Which of the following household liquids would be effective in treating someone with too much acid in the stomach?

C. Milk of Magnesia with pH 10.5

A. Strong salt solution with pH 7.0

C. Milk of Magnesia with pH 10.5

The reaction represented by the equation: Ni$^{2+}$$(_{aq}$) + Fe$(_s$) → Ni$(_s$) + Fe$^{2+}$$(_{aq}$) is a redox reaction because

D. Ni$^{2+}$ ions are reduced and Fe acts as a reducing agent.

A. Ni$^{2+}$ ions are oxidized and Fe acts as a oxidizing agent

B. Ni$^{2+}$ ions are oxidized and Fe acts as a reducing agent

C. Ni$^{2+}$ ions are reduced and Fe acts as an oxidizing agent

D. Ni$^{2+}$ ions are reduced and Fe acts as a reducing agent.

C. turn litmus paper from red to blue

B. turn litmus paper from blue to red

C. turn litmus paper from red to blue

Tetraoxosulphate (VI) acid is considered as a heavy chemical because

B. a high tonnage is produced every year

A. its relative molecular mass is high

B. a high tonnage is produced every year

D. it is used to manufacture heavy chemicals

Aluminium is suitable for making alloys for aircraft construction because it

B. is light and very resistant to corrosion

C. has high density and also a non-conductor of electricity

D. is amphoteric and allotropic

Which of the following statements describes transition elements? They

C. possess variable oxidation state

According to the collision theory of reaction rates, which of the following conditions is not required for two molecules to react? The molecules must

A. come into contact without loss of energy on colliding with each other

B. collide with enough energy to overcome the activation energy barrier

C. collide in an orientation that makes formation of product possible

D. possess enough speed to overcome intermolecular forces of attraction

The electron configuration of $_{29}$ Cu is

A. 1S$^2$ 2S$^2$ 2P$^6$ 3S$^2$ 3P$^6$ 4S$^1$ 3d$^{10}$

B. 1S$^2$ 2S$^2$ 2P$^6$ 3S$^2$ 3P$^6$ 4S$^3$ 3d$^9$

C. 1S$^2$ 2S$^2$ 2P$^5$ 3S$^2$ 3P$^6$ 4S$^2$ 3d$^{10}$

D. 1S$^2$ 2S$^2$ 2P$^6$ 3S$^2$ 3P$^2$ 4S$^1$ 3d$^{10}$

A weak acid is one which

C. does not produce salt with alkali

Catalysts alter reaction rates by

A. providing an alternative reaction pathway

B. lowering the energy of reaction

C. increasing the surface area of reactants

D. aligning the reactant molecules properly

Which of the following statements about ammonium salt is correct? It

B. dissolves in water to form solution of pH < 7

A. dissolves in water to form solution of pH > 1

B. dissolves in water to form solution of pH < 7

C. does not decompose on heating

In ethanol, the attractive forces between adjacent molecules are

C. hydrogen bonding and Van der Waal's forces

How do I prepare for WAEC (WASSCE) Chemistry?

Practise past questions under timed conditions, focus on your weakest topics, and go back over every wrong answer until you understand why the right option is right. DeeepSolve gives you timed CBT-style practice, and you can ask the AI tutor to explain any question step by step.

WAEC (WASSCE) 2024

2024 WAEC (WASSCE) Chemistry Past Questions and Answers

The 2024 WAEC (WASSCE) Chemistry past questions with answers: 50 objective questions from the 2024 paper in the DeeepSolve question bank, each with its answer behind "Show answer". Try each question before you check, then practise the paper as a timed WAEC-style CBT.

Practise this paper as a timed WAEC-style CBT All WAEC (WASSCE) Chemistry years

Questions

1.
Which of the following household liquids would be effective in treating someone with too much acid in the stomach?
1. A.Strong salt solution with pH 7.0
2. B.Tomato juice with pH 4.1
3. C.Milk of Magnesia with pH 10.5
4. D.Black coffee with pH 5.0
Show answer
Answer: C. Milk of Magnesia with pH 10.5
AI explanation
Step 1: Analyze the question Too much stomach acid (hyperacidity) is treated with an antacid, which is a substance that neutralizes excess acid. Antacids are bases (alkaline), so we need the option with a pH well above 7.
Step 2: Explain the concept
pH below 7 means acidic.
pH of 7 means neutral.
pH above 7 means basic (alkaline).
Neutralization: Acid + Base → Salt + Water
Stomach acid is hydrochloric acid (HCl), with a pH of about 1 to 3. To reduce it, we need a mild base that reacts with the HCl.
Step 3: Evaluate each option
A. Salt solution, pH 7.0: Neutral, so it cannot neutralize acid.
B. Tomato juice, pH 4.1: Acidic, so it would add more acid.
C. Milk of Magnesia, pH 10.5: Basic. It contains magnesium hydroxide, Mg(OH)₂, which neutralizes HCl:
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
D. Black coffee, pH 5.0: Acidic, so it would not help and may worsen the problem.
Step 4: Answer The correct option is C. Milk of Magnesia with pH 10.5.
Exam tip: For "treating excess stomach acid" questions, look for the base (pH above 7). Remember that Milk of Magnesia = Mg(OH)₂ = antacid. Other common antacids are sodium hydrogen carbonate (baking soda) and aluminium hydroxide. Note that a strongly basic substance (like NaOH) would be harmful, so antacids are always mild bases.
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2.
When an element exists in two or more forms in the same physical state, it exhibits
1. A.isotopy
2. B.allotropy
3. C.isobars
4. D.isomerism
Show answer
Answer: B. allotropy
AI explanation
Step 1: Analyze the question The question asks for the term used when one element exists in two or more different forms in the same physical state (e.g., all solids).
Step 2: Explain the concept This is allotropy. Allotropes are different structural forms of the same element, in the same physical state, with different physical properties and often similar chemical properties.
Examples:
Carbon: diamond, graphite, fullerene (all solids)
Oxygen: O₂ and ozone (O₃), both gases
Sulfur: rhombic and monoclinic sulfur (both solids)
Phosphorus: red and white phosphorus
Step 3: Check each option
A. Isotopy: Atoms of the same element with the same number of protons but different numbers of neutrons (e.g., ¹²C and ¹⁴C). This concerns atomic structure, not different structural forms of the element.
B. Allotropy: Same element, different forms, same physical state. This matches the question.
C. Isobars: Atoms of different elements with the same mass number but different atomic numbers (e.g., ⁴⁰Ar and ⁴⁰Ca).
D. Isomerism: Compounds, mainly organic, with the same molecular formula but different structural arrangements (e.g., butane and methylbutane). It applies to compounds, not elements.
Step 4: Answer The correct option is B. allotropy.
Tip: "Allo" means "other," so allotropy means "another form." Link it to carbon: diamond and graphite. Also remember that isotopes differ in neutrons, isobars share mass number, and isomers are compounds with the same formula.
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3.
Ethane of volume 400 cm $^3$ was completely burnt in excess oxygen according to the following equation: 2C $_2$ H $_6$ $_(g)$ + 7O $_2$ $_(g)$ → 6H $_2$ O $_(g)$ + 4CO $_2$ $_(g)$ Calculate the volume of steam that would be produced.
1. A.200 cm $^3$
2. B.400 cm $^3$
3. C.600 cm $^3$
4. D.1200 cm $^3$
Show answer
Answer: D. 1200 cm $^3$
AI explanation
Analyze the question: This is a gas volume problem. By Gay-Lussac's law, gases measured at the same temperature and pressure react in volumes that are in the same ratio as their mole ratio in the balanced equation. Steam (H₂O gas) counts as a gas here.
Concept: The coefficients in the equation give the volume ratio directly:
2C₂H₆ : 6H₂O 2 volumes of ethane produce 6 volumes of steam, so the ratio is 1 : 3.
Solution:
2 volumes of C₂H₆ give 6 volumes of H₂O
400 cm³ of C₂H₆ gives x cm³ of H₂O
x = (6 / 2) × 400 = 3 × 400 = 1200 cm³
Check: 400 cm³ is 200 × 2, so the volume scale factor is 200. Steam is 6 × 200 = 1200 cm³.
Tip: For gas volume questions, don't convert to moles. Use the coefficients as volume ratios, and write "given volume × (wanted coefficient ÷ given coefficient)". Always check that the gases are at the same temperature and pressure. Here the oxygen is in excess, so ethane is the limiting reactant and sets the amount of steam.
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4.
The product formed when concentrated sodium chloride solution is electrolysed using carbon electrodes is
1. A.chloride water
2. B.hydrochloric acid
3. C.sodium hydroxide
4. D.sodium oxochlorate (I)
Show answer
Answer: C. sodium hydroxide
AI explanation
Analyzing the question: We are electrolysing concentrated sodium chloride solution (brine) with inert carbon electrodes. We need the product formed.
Concept: In the electrolysis of brine, the solution contains four ions:
From NaCl: Na⁺ and Cl⁻
From water: H⁺ and OH⁻
Selective discharge decides which ions are discharged:
At the cathode (–): H⁺ and Na⁺ compete. H⁺ is lower in the electrochemical series, so it is discharged: 2H⁺ + 2e⁻ → H₂(g)
At the anode (+): Cl⁻ and OH⁻ compete. Because the solution is concentrated, Cl⁻ is preferentially discharged: 2Cl⁻ → Cl₂(g) + 2e⁻
What remains in solution: Na⁺ and OH⁻ ions, which means sodium hydroxide (NaOH) is formed.
Overall: 2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂
Checking the options:
A. Chlorine water: this would need Cl₂ dissolved in water. The Cl₂ escapes as a gas, so it is not the product left in the cell.
B. Hydrochloric acid: H⁺ and Cl⁻ are both discharged, so HCl is not formed.
C. Sodium hydroxide: Na⁺ and OH⁻ remain in solution. ✔
D. Sodium oxochlorate(I) (NaOCl): this forms only if Cl₂ is allowed to mix with cold NaOH, such as in a cell without separation, and it is not the direct product.
Answer: The products are hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in solution.
Tip: Remember "concentrated brine gives H₂, Cl₂, and NaOH." If the solution is dilute, OH⁻ is discharged instead and O₂ is produced at the anode. The concentration of the solution tells you which ion wins at the anode.
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5.
Water pipes are produced from
1. A.polyethene
2. B.Perspex
3. C.polystyrene
4. D.poly vinyl chloride
Show answer
Answer: D. poly vinyl chloride
AI explanation
Step 1: Analyze the question The question asks which polymer is used to make water pipes. This is about the uses of common synthetic polymers.
Step 2: Review the options
A. Polyethene (polythene): Used for plastic bags, films, bowls, buckets, and packaging. A type called HDPE is used for some pipes, but it is mainly known for bags and containers.
B. Perspex (polymethyl methacrylate): A clear, glass-like plastic used for windows, lenses, and signs.
C. Polystyrene: Used for packaging foam, disposable cups, and insulation.
D. Polyvinyl chloride (PVC): Rigid, durable, and resistant to water and corrosion. It is the standard material for water pipes, drainage pipes, and electrical conduit. It is also used for insulation of electric cables, floor tiles, and raincoats.
Step 3: Solve In the WASSCE syllabus, the polymer linked with pipes is PVC. Its rigid, strong, and non-corroding nature makes it ideal for plumbing, unlike metal pipes that rust.
Answer: D. Poly vinyl chloride
Exam tip: Memorize the polymer uses with this quick list:
PVC → Pipes, Vinyl floor tiles, cable coating
Polyethene → bags, bottles, bowls
Polystyrene → foam packaging, cups
Perspex → clear sheets (glass substitute)
Nylon → ropes, fabrics
Teflon (PTFE) → non-stick pans
Think "PVC = Pipes" and you won't forget it.
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6.
The reaction represented by the equation: Ni $^{2+}$ $(_{aq}$ ) + Fe $(_s$ ) → Ni $(_s$ ) + Fe $^{2+}$ $(_{aq}$ ) is a redox reaction because
1. A.Ni $^{2+}$ ions are oxidized and Fe acts as a oxidizing agent
2. B.Ni $^{2+}$ ions are oxidized and Fe acts as a reducing agent
3. C.Ni $^{2+}$ ions are reduced and Fe acts as an oxidizing agent
4. D.Ni $^{2+}$ ions are reduced and Fe acts as a reducing agent.
Show answer
Answer: D. Ni $^{2+}$ ions are reduced and Fe acts as a reducing agent.
AI explanation
Step 1: Analyze the question We need to identify what happens to Ni²⁺ and Fe, and which role each plays (oxidizing or reducing agent).
Step 2: Concept
Oxidation = loss of electrons (oxidation number increases).
Reduction = gain of electrons (oxidation number decreases).
A reducing agent is oxidized itself, because it donates electrons.
An oxidizing agent is reduced itself, because it accepts electrons.
Memory aid: OIL RIG (Oxidation Is Loss, Reduction Is Gain).
Step 3: Solution
Split into half-equations:
Ni²⁺(aq) + 2e⁻ → Ni(s) Oxidation number of Ni goes from +2 to 0. Ni²⁺ gains electrons, so it is reduced. This makes Ni²⁺ the oxidizing agent.
Fe(s) → Fe²⁺(aq) + 2e⁻ Oxidation number of Fe goes from 0 to +2. Fe loses electrons, so it is oxidized. This makes Fe the reducing agent.
Both oxidation and reduction occur at the same time, which is why this is a redox reaction.
Step 4: Check the options
A: Ni²⁺ oxidized, Fe oxidizing agent. Wrong on both counts.
B: Ni²⁺ oxidized. Wrong, since it gains electrons.
C: Ni²⁺ reduced is correct, but Fe as an oxidizing agent is wrong.
D: Ni²⁺ reduced and Fe a reducing agent. Correct.
Tip: The species that is oxidized is always the reducing agent, and the species that is reduced is always the oxidizing agent. Find which one loses electrons (OIL RIG) and the rest follows. Also, a metal going to its ion (like Fe → Fe²⁺) is always oxidation.
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7.
How many isomers can be formed from a compound with molecular formula C $_5$ H $_{12}$ ?
1. A.One
2. B.Two
3. C.Three
4. D.Four
Show answer
Answer: C. Three
AI explanation
Analyze the question: C₅H₁₂ is an alkane (CₙH₂ₙ₊₂ with n = 5). We need to count the different structural arrangements of the carbon atoms.
Concept: Isomers have the same molecular formula but different structural formulas. For alkanes, isomers arise from branching of the carbon chain.
Solution:
Straight chain: n-pentane CH₃–CH₂–CH₂–CH₂–CH₃
One methyl branch on a 4-carbon chain: 2-methylbutane (isopentane) CH₃–CH(CH₃)–CH₂–CH₃
Two methyl branches on a 3-carbon chain: 2,2-dimethylpropane (neopentane) CH₃–C(CH₃)₂–CH₃
Other arrangements, such as 3-methylbutane, are just 2-methylbutane numbered from the other end, so they are not new isomers.
So C₅H₁₂ has three structural isomers in total: n-pentane, 2-methylbutane, and 2,2-dimethylpropane.
Tip: Remember the pattern for alkane isomers: C₄H₁₀ has 2, C₅H₁₂ has 3, C₆H₁₄ has 5, C₇H₁₆ has 9. To find them, start with the straight chain, then shorten the main chain one carbon at a time and place the branches in every distinct position, avoiding duplicates.
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8.
Which of the following arrangements of elements is in order of increasing ionization energy?
1. A.Si, Al, S, P
2. B.S, P, Si, Al
3. C.Al, Si, P, S
4. D.P, Si, S, Al
Show answer
Answer: C. Al, Si, P, S
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9.
The branch of science that deals with the nature and properties of substances and how one substance can be converted to another is known as
1. A.biology
2. B.chemistry
3. C.geography
4. D.physics
Show answer
Answer: B. chemistry
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10.
The oxidation number of iron, in its free state is
1. A.0
2. B.+1
3. C.+2
4. D.+3
Show answer
Answer: A. 0
AI explanation
Step 1: Analyze the question We are asked for the oxidation number of iron (Fe) in its free state. This means iron as an uncombined element, not part of any compound or ion.
Step 2: The concept One of the basic rules for assigning oxidation numbers is:
> The oxidation number of any element in its free (uncombined) state is zero.
This applies whether the element is a single atom (Fe, Na, Mg, Cu) or a molecule made of identical atoms (O₂, H₂, Cl₂, N₂, S₈). No electrons are being gained, lost, or shared unequally, so the oxidation number is 0.
Step 3: Apply the rule
Fe in its free state is just Fe(s), a metal made of neutral iron atoms.
It has not lost or gained any electrons.
So its oxidation number = 0.
Step 4: Check the other options
+1: not a common iron oxidation state.
+2: this is iron in Fe²⁺ compounds, such as FeSO₄ (iron(II) sulphate).
+3: this is iron in Fe³⁺ compounds, such as Fe₂O₃ (iron(III) oxide, the main component of rust).
These apply to iron in combined form, not the free state.
Answer: A. 0
Exam tip: Whenever you see "free state," "elemental form," or "uncombined," think zero. This works for every element, including O₂, H₂, Cu, and Zn. Also remember that the oxidation numbers in a neutral compound add up to zero, while in an ion they add up to the charge on the ion.
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11.
Pairs of outermost shell electrons which are not used in bonding are
1. A.lone pairs
2. B.bonding pairs
3. C.valence electrons
4. D.electrovalent electrons
Show answer
Answer: A. lone pairs
AI explanation
Step 1: Analyze the question We need the term for pairs of electrons in the outermost shell that are not involved in bonding.
Step 2: Explain the concept In covalent bonding, outer-shell electrons fall into two groups:
Bonding pairs (shared pairs): electrons shared between two atoms to form a covalent bond.
Lone pairs (non-bonding pairs): pairs of outer-shell electrons that stay on one atom and are not shared.
Step 3: Examine the options
A. Lone pairs: Correct. These are pairs of valence electrons not used in bonding.
B. Bonding pairs: These are used in bonding, so this is the opposite of what the question asks.
C. Valence electrons: This refers to all the electrons in the outermost shell, whether bonding or not. It is not specific to unshared pairs.
D. Electrovalent electrons: This is not a standard term. "Electrovalent" refers to ionic bonding, which involves electron transfer.
Step 4: Example In ammonia (NH₃), nitrogen has 5 valence electrons. Three are used to form three N–H bonds, leaving one lone pair on nitrogen. In water (H₂O), oxygen has two lone pairs. These lone pairs affect molecular shape, which is why NH₃ is pyramidal and H₂O is bent.
Answer: A. lone pairs
Exam tip: Remember "lone = alone." A lone pair sits alone on one atom, while a bonding pair is shared between two atoms.
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12.
Bases normally
1. A.are corrosive
2. B.turn litmus paper from blue to red
3. C.turn litmus paper from red to blue
4. D.are non-metal oxides
Show answer
Answer: C. turn litmus paper from red to blue
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13.
Which of the following gases could be collected by downward displacement of air?
1. A.Chlorine
2. B.Hydrogen
3. C.hydrogen chloride
4. D.Sulphur(IV) oxide
Show answer
Answer: B. Hydrogen
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14.
The number of shared pair of electrons in a molecule of methane is
1. A.2
2. B.4
3. C.6
4. D.8
Show answer
Answer: B. 4
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15.
The volume of 22g of CO $_2$ at stp is equivalent to [C =12, O =16, molar volume at stp = 22.4 dm $^3$ ]
1. A.22.0 dm $^3$
2. B.22.4 dm $^3$
3. C.11.2 dm $^3$
4. D.5.6 dm $^3$
Show answer
Answer: C. 11.2 dm $^3$
AI explanation
Analyze the question: We need the volume occupied by 22 g of CO₂ at STP, using the molar volume of 22.4 dm³.
Concept: One mole of any gas occupies 22.4 dm³ at STP.
moles = mass ÷ molar mass, and volume at STP = moles × 22.4 dm³
Solution:
Molar mass of CO₂ = 12 + (2 × 16) = 44 g/mol
Moles of CO₂ = 22 ÷ 44 = 0.5 mol
Volume = 0.5 × 22.4 = 11.2 dm³
Answer: The volume is 11.2 dm³, which is option C.
Tip: Check how 22 g compares with the molar mass. Here 22 g is exactly half of 44 g, so the volume is half of 22.4 dm³. Spotting simple fractions like this saves time in the exam.
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16.
The metallic bonding in aluminium is strong because of large number of
1. A.delocalized electrons
2. B.immobile electrons
3. C.lone pair electrons
4. D.valence electrons
Show answer
Answer: A. delocalized electrons
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17.
Ethanedioic acid is an organic solid that can be purified by
1. A.decantation
2. B.distillation
3. C.crystallization
4. D.filtration
Show answer
Answer: C. crystallization
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18.
Isotopes of the same element have similar chemical properties because they have the same number of
1. A.nuclides
2. B.protons
3. C.neutrons
4. D.atoms
Show answer
Answer: B. protons
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19.
How many covalent bonds are formed by Nitrogen?
1. A.1
2. B.2
3. C.3
4. D.4
Show answer
Answer: C. 3
AI explanation
Analyze the question: It asks how many covalent bonds a nitrogen atom typically forms.
Concept: An atom forms covalent bonds by sharing electrons until it has a stable octet (8 electrons in its outer shell). The number of bonds it forms is usually equal to the number of electrons it needs to complete that octet.
Solution:
Nitrogen has atomic number 7, so its electron configuration is 2, 5 (1s² 2s² 2p³).
It has 5 valence electrons.
To reach an octet, it needs 8 − 5 = 3 more electrons.
It does this by sharing 3 electrons, forming 3 covalent bonds.
Examples:
Ammonia, NH₃: nitrogen forms 3 single bonds with three hydrogen atoms, and keeps one lone pair.
Nitrogen gas, N₂: the two atoms share 3 pairs of electrons, giving a triple bond (N≡N).
Note on option D: Nitrogen can reach 4 bonds in the ammonium ion (NH₄⁺), but the fourth bond is a dative (coordinate) covalent bond, where nitrogen donates its lone pair. The normal number of covalent bonds nitrogen forms is 3.
Tip: Use the "8 minus group number" rule for non-metals. Nitrogen is in Group 5, so 8 − 5 = 3 bonds. Likewise, oxygen (Group 6) forms 2 bonds, and chlorine (Group 7) forms 1.
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20.
The following organic compounds are polymers except
1. A.rubber
2. B.starch
3. C.proteins
4. D.fats
Show answer
Answer: D. fats
AI explanation
Analyze the question: We are asked which compound is NOT a polymer. A polymer is a large molecule made from many repeating small units (monomers) joined together.
Explain the concept:
Rubber (natural rubber) is a polymer of isoprene units (polyisoprene).
Starch is a polymer of glucose units joined by glycosidic bonds (a polysaccharide).
Proteins are polymers of amino acids joined by peptide bonds.
Fats are esters formed from one glycerol molecule and three fatty acid molecules (triglycerides). They are large molecules, but they are not made of long chains of repeating monomer units, so they are not true polymers.
Step-by-step solution:
Rubber: repeating isoprene units, so it is a polymer.
Starch: repeating glucose units, so it is a polymer.
Proteins: repeating amino acid units, so it is a polymer.
Fats: a single glycerol joined to three fatty acids, with no repeating chain, so it is not a polymer.
Answer: D. fats
Exam tip: Remember the monomer pairs: rubber → isoprene, starch/cellulose → glucose, proteins → amino acids, polythene → ethene. Fats and oils are esters (triglycerides), not polymers. When you see "except" in a polymer question, look for fats and oils.
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21.
The main function of limestone in the blast furnace during the extraction of iron is to
1. A.act as catalyst
2. B.remove impurities
3. C.act as a reducing agent
4. D.supply Carbon(IV) oxide
Show answer
Answer: B. remove impurities
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22.
Tetraoxosulphate (VI) acid is considered as a heavy chemical because
1. A.its relative molecular mass is high
2. B.a high tonnage is produced every year
3. C.it is an inorganic chemical
4. D.it is used to manufacture heavy chemicals
Show answer
Answer: B. a high tonnage is produced every year
AI explanation
Analyze the question: It asks why sulphuric acid (H₂SO₄, tetraoxosulphate(VI) acid) is classed as a "heavy chemical."
Explain the concept: In industrial chemistry, "heavy chemicals" are not defined by molecular mass or weight. They are chemicals produced in very large quantities (thousands to millions of tonnes per year) by industry. Examples include sulphuric acid, sodium hydroxide, sodium carbonate, ammonia, and nitric acid. They are usually cheap, impure or bulk-grade, and serve as raw materials for making other products. "Fine chemicals," by contrast, are made in small quantities and are highly pure, such as pharmaceuticals.
Solution (checking each option):
A. Its relative molecular mass is high. False. H₂SO₄ has a molar mass of 98 g/mol, which is not especially high, and molecular mass is not the criterion anyway.
B. A high tonnage is produced every year. True. Sulphuric acid is among the most widely produced industrial chemicals in the world, used in fertilizers (superphosphate), detergents, paints, batteries, and more.
C. It is an inorganic chemical. Being inorganic does not make a chemical "heavy." Many inorganic chemicals are made in small amounts.
D. It is used to manufacture heavy chemicals. Not the defining reason. Heavy chemicals are defined by the scale of their own production.
Tip: "Heavy" in "heavy chemicals" refers to tonnage (quantity produced), not weight or molecular mass. Whenever you see "heavy chemical" in WASSCE, JAMB, or NECO, think large-scale production.
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23.
Aluminium is suitable for making alloys for aircraft construction because it
1. A.is hard and brittle
2. B.is light and very resistant to corrosion
3. C.has high density and also a non-conductor of electricity
4. D.is amphoteric and allotropic
Show answer
Answer: B. is light and very resistant to corrosion
AI explanation
Step 1: Analyze the question We need the property of aluminium that makes its alloys good for building aircraft. Aircraft materials must be light (low density), strong, and able to resist corrosion from air and moisture.
Step 2: Explain the concept Aluminium is a light metal with a density of about 2.7 g/cm³, much lower than iron (about 7.9 g/cm³). It also forms a thin, tough, unreactive layer of aluminium oxide (Al₂O₃) on its surface when exposed to air. This layer protects the metal underneath from further corrosion. Alloys such as duralumin (Al with Cu, Mg, Mn) add strength while keeping the metal light.
Step 3: Evaluate each option
A. Hard and brittle: Wrong. Brittle materials crack easily, which is dangerous in aircraft. Aluminium is ductile and malleable.
B. Light and very resistant to corrosion: Correct. Both properties suit aircraft construction.
C. High density and a non-conductor of electricity: Wrong. Aluminium has low density and is a good conductor of electricity.
D. Amphoteric and allotropic: Aluminium is amphoteric (its oxide reacts with both acids and bases), but this is not why it is used in aircraft. It is also not allotropic.
Step 4: Answer The correct option is B. is light and very resistant to corrosion.
Tip: For "why is metal X used for Y" questions, match the property to the use. Aircraft need lightness and corrosion resistance. Electric cables need conductivity and ductility. Cooking pots need good heat conduction and non-toxicity. Also remember that "brittle" is almost never the right answer for a construction material.
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24.
Which of the following statements describes transition elements? They
1. A.are very reactive
2. B.have low melting points
3. C.possess variable oxidation state
4. D.form colourless salts
Show answer
Answer: C. possess variable oxidation state
AI explanation
Step 1: Analyze the question You are asked to pick the statement that correctly describes transition elements (the d-block elements, e.g. Fe, Cu, Mn, Cr, Ni, Zn-Cu series).
Step 2: Key concept Transition elements have partially filled d-orbitals. Because the 4s and 3d electrons are very close in energy, they can lose different numbers of electrons. This gives rise to these characteristic properties:
Variable oxidation states (e.g. Fe²⁺ and Fe³⁺, Mn²⁺ to Mn⁷⁺)
Formation of coloured compounds/ions (e.g. Cu²⁺ is blue, Fe²⁺ is green, Fe³⁺ is yellow-brown)
Catalytic activity (e.g. Fe in the Haber process, V₂O₅ in the Contact process, MnO₂)
Formation of complex ions
Paramagnetism
High melting points and densities, and they are hard, strong metals
Step 3: Evaluate each option
A. Very reactive: Wrong. Transition metals are generally much less reactive than Group 1 and 2 metals.
B. Low melting points: Wrong. They typically have high melting points (e.g. iron melts at about 1538 °C).
C. Possess variable oxidation state: Correct. This is a hallmark property of transition elements.
D. Form colourless salts: Wrong. They typically form coloured salts (e.g. CuSO₄ is blue, FeSO₄ is green).
Step 4: Answer
Tip: Remember the transition element "VCC" properties: Variable oxidation states, Coloured compounds, Catalytic activity. If an option says "variable oxidation state," "coloured," or "catalyst," it is probably right. If it says "low melting point" or "colourless," eliminate it.
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25.
According to the collision theory of reaction rates, which of the following conditions is not required for two molecules to react? The molecules must
1. A.come into contact without loss of energy on colliding with each other
2. B.collide with enough energy to overcome the activation energy barrier
3. C.collide in an orientation that makes formation of product possible
4. D.possess enough speed to overcome intermolecular forces of attraction
Show answer
Answer: A. come into contact without loss of energy on colliding with each other
AI explanation
Analyze the question: We need the condition that is NOT required by collision theory for a reaction to occur.
Concept: Collision theory. For reactant particles to form products, three things are needed:
The particles must collide.
They must collide with energy equal to or greater than the activation energy (Ea).
They must collide with the correct orientation (the steric factor).
Evaluating each option:
A. Come into contact without loss of energy on colliding. This is not a requirement. In a real collision, kinetic energy is converted into potential energy (bond stretching and breaking) as the molecules react. Energy is transferred during the collision, so "no loss of energy" is not a condition. The molecules need enough energy at the moment of collision, not zero energy loss. So this is the condition that is not required.
B. Enough energy to overcome the activation energy barrier. This is required, so it is not the answer.
C. Correct orientation. This is required, so it is not the answer.
D. Enough speed to overcome intermolecular forces of attraction. This is a loosely worded statement. It is related to having sufficient kinetic energy to collide effectively, and it is the weaker of the "required" statements. However, it is not part of the standard three-condition collision theory.
Note on D: Option D is poorly phrased, and some would argue it is not a standard condition either. But A is the better answer, since it directly contradicts what happens in an effective collision. Energy is converted during bond breaking, so it is not "lost" or preserved in any required way.
Answer: A
Tip: Remember the collision theory as "C-E-O": Collision, Energy (≥ Ea), Orientation. Anything outside these three, like "no energy loss," is usually the odd one out in "which is NOT required" questions.
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26.
Which of the following substances has the lowest boiling point?
1. A.Aqueous sodium chloride
2. B.Ethanol
3. C.Tetrachloromethane
4. D.Water
Show answer
Answer: C. Tetrachloromethane
AI explanation
Analyze the question: We need the substance with the lowest boiling point. That depends on the strength of the forces between its molecules.
Concept: Boiling point depends on intermolecular forces (and ions in solution):
Hydrogen bonding is the strongest of the molecular forces, so it gives high boiling points.
Van der Waals (London dispersion) forces are weak, so they give low boiling points.
Dissolved solutes raise the boiling point of a solvent (boiling point elevation).
Step-by-step:
| Option | Substance | Main forces | Boiling point | |---|---|---|---| | A | Aqueous NaCl | Hydrogen bonding plus dissolved ions | Above 100 °C | | B | Ethanol | Hydrogen bonding (O-H) | about 78 °C | | C | Tetrachloromethane (CCl₄) | Only weak van der Waals forces (non-polar, symmetrical) | about 77 °C | | D | Water | Extensive hydrogen bonding | 100 °C |
Checking the close call: ethanol boils at about 78 °C and CCl₄ at about 76.7 °C. CCl₄ is slightly lower, so it has the lowest boiling point. Its heavy mass gives it some dispersion forces, but it has no hydrogen bonding, so it is still the lowest here. The reasoning on forces points the same way: it is the only substance with no hydrogen bonding.
Answer: C, Tetrachloromethane.
Exam tip: Rank boiling points by asking "Is there hydrogen bonding?" Substances with it (water, alcohols) boil higher than non-polar covalent compounds (CCl₄, hydrocarbons). Remember that solutions boil higher than pure solvents. Note that ethanol and CCl₄ are close in value, so rely on the type of intermolecular force rather than memorized numbers alone.
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27.
The percentage by mass of Oxygen in MgSO $_4$ .7H $_2$ O is [ M $_r$ of MgSO $_4$ .7H $_2$ O = 246 ]
1. A.26.0%
2. B.45.5%
3. C.71.5%
4. D.84.0%
Show answer
Answer: C. 71.5%
AI explanation
Analyze the question: We need the percentage by mass of oxygen in hydrated magnesium sulfate, MgSO₄·7H₂O, with Mr = 246.
Concept: Percentage of an element = (total mass of that element in one formula unit ÷ molar mass of the compound) × 100
Solution:
Count the oxygen atoms:
In MgSO₄: 4 oxygen atoms
In 7H₂O: 7 oxygen atoms
Total = 11 oxygen atoms
Mass of oxygen = 11 × 16 = 176
Percentage of oxygen = (176 ÷ 246) × 100 = 71.5%
Check the Mr: Mg (24) + S (32) + 4O (64) = 120. 7H₂O = 7 × 18 = 126. Total = 246 ✔
Answer: The percentage of oxygen is 71.5%, which is option C.
Common mistakes:
Option B (45.5%) is not what you get if you forget the water: counting only 4 oxygens gives 64/246 = 26.0% (option A, a trap).
Option D (84.0%) is not obtained by any correct approach here.
Tip: For hydrated salts, always count the oxygen from the water of crystallization too. Write the total number of atoms first (here 4 + 7 = 11) before calculating, so you do not miss any.
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28.
When s and p block elements react, the bond formed is
1. A.electrovalent
2. B.covalent
3. C.metallic
4. D.dative-covalent
Show answer
Answer: A. electrovalent
AI explanation
Step 1: Analyze the question The question asks what type of bond forms when an s-block element (e.g., Na, K, Mg, Ca) reacts with a p-block element (e.g., Cl, O, S, N).
Step 2: Explain the concept
s-block elements (Groups 1 and 2) are metals with low ionization energies, so they easily lose their valence electrons.
p-block elements on the right side of the table (Groups 15-17) are non-metals with high electronegativity, so they readily gain electrons.
When electrons are transferred from the metal to the non-metal, oppositely charged ions form and are held together by electrostatic attraction. This is electrovalent (ionic) bonding.
Step 3: Work through an example Sodium (s-block) reacting with chlorine (p-block):
Na → Na⁺ + e⁻ (loses 1 electron)
Cl + e⁻ → Cl⁻ (gains 1 electron)
Na⁺ and Cl⁻ attract to form NaCl, an ionic compound.
Likewise, Mg (s-block) + O (p-block) gives Mg²⁺ and O²⁻, forming MgO.
Step 4: Eliminate the other options
B. Covalent: involves sharing of electrons, typically between two non-metals (p-block with p-block).
C. Metallic: occurs between atoms of metals only, in a lattice of positive ions and delocalized electrons.
D. Dative-covalent: one atom donates both electrons of the shared pair (e.g., NH₄⁺), which is not the typical s-p reaction.
Answer: The bond formed is electrovalent, which is option A.
Exam tip: Remember "metal + non-metal = ionic (electrovalent)", and "non-metal + non-metal = covalent". The s-block is mostly metals and the right-hand p-block is non-metals, so s + p usually means electron transfer. (A few exceptions, like BeCl₂, show covalent character, but WASSCE expects electrovalent here.)
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29.
The product of the reaction between ethanol and excess acidified K $_2$ Cr $_2$ O $_7$ is
1. A.CH $_2$ =CH $_2$
2. B.CH $_3$ OCH $_3$
3. C.CH $_3$ COOH
4. D.CH $_3$ CH $_3$
Show answer
Answer: C. CH $_3$ COOH
AI explanation
Analyze the question
Ethanol is a primary alcohol. Acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺, is a strong oxidising agent. The key phrase is "excess", which tells you how far the oxidation goes.
Concept: oxidation of primary alcohols
A primary alcohol is oxidised in two stages:
Primary alcohol → aldehyde (partial oxidation)
Aldehyde → carboxylic acid (complete oxidation)
If the aldehyde is distilled off as it forms, you stop at the aldehyde.
With excess oxidising agent (usually under reflux), the oxidation continues to the carboxylic acid.
Step-by-step solution
Step 1: Ethanol is oxidised to ethanal.
CH₃CH₂OH → [O] → CH₃CHO + H₂O
Step 2: With excess K₂Cr₂O₇/H⁺, ethanal is oxidised further to ethanoic acid.
CH₃CHO + [O] → CH₃COOH
Overall:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Colour change: the orange Cr₂O₇²⁻ is reduced to green Cr³⁺.
Checking the other options
A. CH₂=CH₂ (ethene) is formed by dehydration of ethanol with conc. H₂SO₄ at 170 °C, not by oxidation.
B. CH₃OCH₃ (methoxymethane) is an isomer of ethanol. It is not formed by oxidation.
D. CH₃CH₃ (ethane) would need reduction, not oxidation.
Correct answer: C. CH₃COOH (ethanoic acid)
Exam tip: Remember "excess oxidant = carboxylic acid". For primary alcohols:
Mild or limited oxidant with distillation gives the aldehyde.
Excess oxidant with reflux gives the carboxylic acid.
Also, secondary alcohols give ketones, and tertiary alcohols resist oxidation.
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30.
Which of the following functional groups is present in alkanoic acid?
1. A.-COOH
2. B.-OH
3. C.-COOR
4. D.-CHO
Show answer
Answer: A. -COOH
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31.
$_8$ X $^{2-}$ and $_{10}$ Y are
1. A.isomers
2. B.isotopes
3. C.allotropes
4. D.isoelectronic
Show answer
Answer: D. isoelectronic
AI explanation
Analyze the question We are given two species: X²⁻ with atomic number 8, and Y with atomic number 10. We need to find how they are related.
Concept
Isomers: compounds with the same molecular formula but different structures.
Isotopes: atoms of the same element (same atomic number) with different mass numbers.
Allotropes: different physical forms of the same element (e.g., diamond and graphite).
Isoelectronic: different species with the same number of electrons.
Solution
X has atomic number 8, so a neutral atom has 8 electrons (it is oxygen).
X²⁻ has gained 2 electrons: 8 + 2 = 10 electrons.
Y has atomic number 10, so a neutral atom has 10 electrons (it is neon).
Both species have 10 electrons, with the configuration 1s² 2s² 2p⁶.
Checking the other options:
They have different atomic numbers (8 and 10), so they are different elements. They cannot be isotopes or allotropes.
They are not compounds with the same molecular formula, so they are not isomers.
Since both have the same number of electrons (10), they are isoelectronic.
Exam tip For "isoelectronic" questions, always compute the electron count: for a negative ion, add the charge to the atomic number; for a positive ion, subtract it. Familiar isoelectronic sets with 10 electrons are N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, and Al³⁺.
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32.
The most important ore of aluminium is
1. A.bauxite
2. B.haematite
3. C.magnetite
4. D.monazite
Show answer
Answer: A. bauxite
AI explanation
Analyze the question: You're asked to identify the main ore from which aluminium is extracted.
Explain the concept: An ore is a naturally occurring mineral from which a metal can be extracted economically. Aluminium is the most abundant metal in the Earth's crust, but it is always found combined with other elements. Its main commercial ore is bauxite, which is hydrated aluminium oxide, Al₂O₃·2H₂O.
Check each option:
A. Bauxite: Al₂O₃·2H₂O. This is the main ore of aluminium. ✔
B. Haematite: Fe₂O₃. This is an ore of iron.
C. Magnetite: Fe₃O₄. This is also an ore of iron.
D. Monazite: a phosphate mineral of rare earth elements and thorium (not aluminium).
How aluminium is extracted:
Bauxite is purified to get pure alumina (Al₂O₃) using the Bayer process.
The alumina is dissolved in molten cryolite (Na₃AlF₆), which lowers the melting point.
The mixture is electrolysed (the Hall-Héroult process) with carbon electrodes, and aluminium collects at the cathode.
Answer: The correct option is A. bauxite.
Exam tip: Match the ore with its metal using these:
Bauxite → Aluminium
Haematite and Magnetite → Iron (both are iron oxides)
Cassiterite → Tin
Galena → Lead
Zinc blende → Zinc
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33.
An atom of an element in the ground state contains 8 valence electrons. The element is considered as a
1. A.metal
2. B.semi-metal
3. C.noble gas
4. D.halogen
Show answer
Answer: C. noble gas
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34.
The electron configuration of $_{29}$ Cu is
1. A.1S $^2$ 2S $^2$ 2P $^6$ 3S $^2$ 3P $^6$ 4S $^1$ 3d $^{10}$
2. B.1S $^2$ 2S $^2$ 2P $^6$ 3S $^2$ 3P $^6$ 4S $^3$ 3d $^9$
3. C.1S $^2$ 2S $^2$ 2P $^5$ 3S $^2$ 3P $^6$ 4S $^2$ 3d $^{10}$
4. D.1S $^2$ 2S $^2$ 2P $^6$ 3S $^2$ 3P $^2$ 4S $^1$ 3d $^{10}$
Show answer
Answer: A. 1S $^2$ 2S $^2$ 2P $^6$ 3S $^2$ 3P $^6$ 4S $^1$ 3d $^{10}$
AI explanation
Step 1: Analyze the question We need the ground-state electron configuration of copper (Z = 29), so it has 29 electrons.
Step 2: The concept Electrons fill orbitals in this order (Aufbau principle): 1s → 2s → 2p → 3s → 3p → 4s → 3d. Each subshell holds a limited number of electrons: s = 2, p = 6, d = 10. Copper is a well-known exception. A completely filled 3d subshell (d¹⁰) is extra stable, so one electron moves from 4s to 3d.
Step 3: Solution
Expected: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁹
Actual (the exception): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
Check the total: 2 + 2 + 6 + 2 + 6 + 1 + 10 = 29 ✓
Step 4: Check the options
A: 2+2+6+2+6+1+10 = 29 ✓ and it matches copper's configuration.
B: 4s³ is impossible, because an s subshell holds a maximum of 2 electrons. ✗
C: 2p⁵ gives a total of 2+2+5+2+6+2+10 = 29, but 2p is not filled before 3s begins, so it is not the ground state. ✗
D: 3p² gives a total of 2+2+6+2+2+1+10 = 25, not 29. ✗
Answer: A
Exam tip: Remember the two famous exceptions, Cr (3d⁵ 4s¹) and Cu (3d¹⁰ 4s¹). Half-filled and fully filled d subshells are extra stable. Also, always add up the superscripts to check they equal the atomic number, which quickly eliminates wrong options.
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35.
Consider the reaction represented by the equation: C $_3$ H $_8$ + 5 O $_2$ → 3CO $_2$ + 4H $_2$ O If 0.1 mole of C $_3$ H $_8$ was completely burnt, what volume of CO $_2$ would be produced at stp? [volume of gas at stp = 22.4dm $^3$ mol $^{-1}$ ].
1. A.6.72dm $^3$
2. B.2.24dm $^3$
3. C.0.30 dm $^3$
4. D.0.10dm $^3$
Show answer
Answer: A. 6.72dm $^3$
AI explanation
Step 1: Analyze the question We are given 0.1 mole of propane (C₃H₈) burnt completely. We need the volume of CO₂ produced at stp. Step 2: Concept The balanced equation gives mole ratios. At stp, 1 mole of any gas occupies 22.4 dm³: Volume = moles × 22.4 dm³/mol Step 3: Solution From the equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O 1 mole of C₃H₈ produces 3 moles of CO₂. So 0.1 mole of C₃H₈ produces: 0.1 × 3 = 0.3 mole of CO₂ Volume of CO₂ = 0.3 × 22.4 = 6.72 dm³ Step 4: Answer The correct option is A. 6.72 dm³. Note that option C (0.30 dm³) is a trap. It is the number of moles of CO₂, not the volume. Always multiply by 22.4 to convert moles to volume. Tip: For gas-volume questions, follow this chain: moles given → mole ratio from the equation → moles wanted → × 22.4 dm³. Check that your final answer is in volume units, not moles.
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36.
Consider the reaction represented by the following equation: x Al + yCl $_2$ → z AlCl $_3$
1. A.2, 3 and 2
2. B.2, 2 and 3
3. C.1, 2 and 1
4. D.1, 1 and 1
Show answer
Answer: A. 2, 3 and 2
AI explanation
Analyze the question: You need to balance the equation x Al + y Cl₂ → z AlCl₃, so that the number of atoms of each element is the same on both sides.
Concept: Balancing means choosing coefficients so that atoms are conserved. Cl₂ is diatomic (2 Cl atoms per molecule), while AlCl₃ has 3 Cl atoms per formula unit. So we need a common multiple of 2 and 3, which is 6.
Step-by-step solution:
Balance Al: x = z
Balance Cl: 2y = 3z
Take the smallest whole numbers. If z = 2, then 2y = 6, so y = 3. Then x = 2.
The balanced equation is:
2Al + 3Cl₂ → 2AlCl₃
Check:
Left: Al = 2, Cl = 3 × 2 = 6
Right: Al = 2, Cl = 2 × 3 = 6 ✔
So x = 2, y = 3, z = 2.
Checking the options:
A. 2, 3 and 2 ✔ (matches the balanced equation)
B. 2, 2 and 3 ✘ (gives 4 Cl on the left and 9 on the right)
C. 1, 2 and 1 ✘ (gives 4 Cl on the left and 3 on the right)
D. 1, 1 and 1 ✘ (gives 2 Cl on the left and 3 on the right)
Exam tip: When one side has a diatomic molecule (like Cl₂, O₂, H₂) and the other has an odd number of that atom, find the LCM. Here the LCM of 2 and 3 is 6, so you need 3 Cl₂ and 2 AlCl₃. Always do a quick atom count to confirm.
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37.
A saturated solution at 30 $^0$ C will normally produce crystals at a temperature of
1. A.50 $^0$ C
2. B.40 $^0$ C
3. C.35 $^0$ C
4. D.20 $^0$ C
Show answer
Answer: D. 20 $^0$ C
AI explanation
Analyze the question: A solution that is saturated at 30°C holds the maximum amount of solute it can dissolve at that temperature. The question asks at which temperature crystals will normally form.
Concept: For most solids (such as KNO₃ and NaNO₃), solubility increases as temperature rises. So:
Heating the solution makes it unsaturated, because more solute can dissolve.
Cooling the solution lowers solubility, so the solution holds more solute than it can keep dissolved. The excess comes out as crystals.
Solution:
At 50°C, 40°C and 35°C, which are all above 30°C, solubility is higher. The solution becomes unsaturated and no crystals form.
At 20°C, which is below 30°C, solubility is lower. The excess solute crystallizes out.
Answer: D (20°C)
Tip: "Cool down, crystals crown." For most solids, cooling a saturated solution produces crystals, and heating it dissolves more solute. Watch for the exception of gases, whose solubility falls as temperature rises.
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38.
Which of the following conditions are necessary for the preparation of alkanoates from alkanols and alkanoic acid?
1. A.Water and NaOH
2. B.Conc. H $_2$ SO $_4$ and heat
3. C.NaOH and heat
4. D.water and aqueous HCl
Show answer
Answer: B. Conc. H $_2$ SO $_4$ and heat
AI explanation
Analyze the question: You are asked for the conditions needed to make an ester (an alkanoate) from an alkanol (alcohol) and an alkanoic acid (carboxylic acid). This reaction is called esterification.
Concept: Esterification is a reversible, condensation reaction:
RCOOH + R'OH ⇌ RCOOR' + H₂O
Concentrated H₂SO₄ acts as a catalyst. It also acts as a dehydrating agent: it absorbs the water formed, which pushes the equilibrium toward the ester.
Heat (reflux) speeds up the reaction, because it is slow at room temperature.
Step-by-step check of the options:
A. Water and NaOH: NaOH is a base. It would neutralise the acid to form a salt and would hydrolyse any ester formed (saponification). Water also pushes the equilibrium backward. ❌
B. Conc. H₂SO₄ and heat: This is the standard catalyst and condition for esterification. ✅
C. NaOH and heat: This is the condition for alkaline hydrolysis of an ester, which is the reverse of what we want. ❌
D. Water and aqueous HCl: This is a dilute acid medium, which favours hydrolysis of esters, not their formation. ❌
Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (conc. H₂SO₄, heat)
Here, ethanoic acid and ethanol give ethyl ethanoate.
Answer: B. Conc. H₂SO₄ and heat
Exam tip: Remember "Esterification = Acid + Alcohol + conc. H₂SO₄ + heat". If you see NaOH or lots of water, think of hydrolysis or saponification, which breaks esters down instead of making them.
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39.
At 25 $^0$ C, the saturated solution of a salt in water was found to contain 0.24g of the salt in 100cm $^3$ . What is the solubility of the salt in gdm $^{-1}$ ?
1. A.0.024
2. B.0.240
3. C.2.40
4. D.24.0
Show answer
Answer: C. 2.40
AI explanation
Analyze the question: The solubility is given as 0.24 g of salt in 100 cm³ of solution. You are asked for it in g/dm³ (the "gdm⁻¹" in the question is a typo for g dm⁻³).
Concept: Solubility in g/dm³ is the mass of solute (in grams) dissolved in 1 dm³ of solution.
1 dm³ = 1000 cm³
Solution:
100 cm³ contains 0.24 g
1000 cm³ will contain (0.24 × 1000) / 100 g
= 240 / 100
= 2.40 g/dm³
Check: 1000 cm³ is 10 times 100 cm³, so the mass must be 10 times larger: 0.24 × 10 = 2.40 g. ✔
Answer: Option C, 2.40 g/dm³.
Tip: To convert from g/100 cm³ to g/dm³, just multiply by 10 (move the decimal point one place to the right). Common mistakes are multiplying by 100 (which gives 24.0, option D) or dividing by 10 (which gives 0.024, option A).
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40.
A weak acid is one which
1. A.is not corrosive
2. B.completely ionizes in water
3. C.does not produce salt with alkali
4. D.slightly ionizes in water
Show answer
Answer: D. slightly ionizes in water
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41.
Catalysts alter reaction rates by
1. A.providing an alternative reaction pathway
2. B.lowering the energy of reaction
3. C.increasing the surface area of reactants
4. D.aligning the reactant molecules properly
Show answer
Answer: A. providing an alternative reaction pathway
AI explanation
Step 1: Analyze the question The question asks how a catalyst changes the rate of a reaction.
Step 2: The concept A catalyst is a substance that changes the rate of a chemical reaction without being used up. It works by providing an alternative reaction pathway (mechanism) with a lower activation energy. More reactant particles then have enough energy to react when they collide, so the rate increases.
Step 3: Evaluate each option
A. Providing an alternative reaction pathway: Correct. This is the standard definition of how catalysts work. The new pathway has a lower activation energy.
B. Lowering the energy of reaction: Misleading. A catalyst lowers the activation energy, not the overall energy change (enthalpy change, ΔH) of the reaction. The reactants and products stay at the same energy levels, so ΔH is unchanged.
C. Increasing the surface area of reactants: This describes what happens when you crush a solid into powder. It is not how a catalyst works.
D. Aligning the reactant molecules properly: Some surface catalysts do help orient molecules, but this is a side detail of certain heterogeneous catalysts. It is not the general explanation that applies to all catalysts.
Step 4: Answer The best and most general answer is A.
Tip: Remember "Catalyst = new path, lower activation energy, same ΔH." If an option says the catalyst changes the overall energy of reaction, be careful. WASSCE examiners often use wording like "energy of reaction" to trap students who forget the difference between activation energy and enthalpy change.
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42.
Which of the following compounds is formed by the oxidation of ethanol?
1. A.C $_2$ H $_4$ CO $_2$ H
2. B.C $_2$ H $_5$ OH
3. C.CH $_3$ OH
4. D.CH $_3$ CO $_2$ H
Show answer
Answer: D. CH $_3$ CO $_2$ H
AI explanation
Analyze the question: You're asked which compound forms when ethanol (C₂H₅OH) is oxidised.
Concept: Primary alcohols oxidise in stages:
Primary alcohol → aldehyde → carboxylic acid
With a strong oxidising agent such as acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) or acidified potassium permanganate (KMnO₄/H⁺), the oxidation goes all the way to the carboxylic acid. Both the alkyl chain and the carbon count stay the same, so the product has 2 carbons.
Solution:
Ethanol: CH₃CH₂OH
First oxidation gives ethanal (an aldehyde): CH₃CHO
Further oxidation gives ethanoic acid (a carboxylic acid): CH₃COOH
Overall: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Checking the options:
A. C₂H₄CO₂H is propanoic acid (3 carbons). Oxidation does not add a carbon, so this is wrong.
B. C₂H₅OH is ethanol itself, the starting material, so this is wrong.
C. CH₃OH is methanol (1 carbon). Oxidation does not remove a carbon, so this is wrong.
D. CH₃CO₂H is ethanoic (acetic) acid, which is correct.
Exam tip: Count the carbons. Oxidation of an alcohol keeps the same number of carbons, and only the functional group changes: –CH₂OH → –CHO → –COOH. This is also why vinegar (ethanoic acid) forms when palm wine or wine is left exposed to air. Bacteria oxidise the ethanol in it.
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43.
The electron configuration 1S $^2$ 2S $^2$ 2P $_x$ $^2$ contravenes the
1. A.Pauli's exclusion principle
2. B.Aufbau's principle
3. C.octet rule
4. D.Hund's rule
Show answer
Answer: D. Hund's rule
AI explanation
Analyze the question
The configuration 1s² 2s² 2pₓ² has 6 electrons (carbon). The two 2p electrons are both placed in the same orbital (2pₓ), even though 2p_y and 2p_z are empty. We need to find which rule this breaks.
Concept: the three rules for filling orbitals
Aufbau principle: electrons fill the lowest-energy orbitals first (1s → 2s → 2p → 3s...).
Pauli's exclusion principle: no two electrons in an atom can have the same four quantum numbers, so an orbital holds a maximum of 2 electrons with opposite spins.
Hund's rule: electrons occupy degenerate orbitals (such as the three 2p orbitals) singly first, with parallel spins, before any pairing begins.
Solution
1s² and 2s² are filled in the correct order, so Aufbau is obeyed.
2pₓ² puts two electrons in one orbital. This is allowed by Pauli, as long as their spins are opposite.
The octet rule concerns bonding and valence shells, not ground-state orbital filling, so it is not relevant here.
The problem is that 2p_y and 2p_z are empty while 2pₓ is doubly occupied. The correct configuration is 1s² 2s² 2pₓ¹ 2p_y¹, with the two electrons unpaired in separate p orbitals. Pairing before every degenerate orbital has one electron breaks Hund's rule.
Answer: D. Hund's rule
Tip: Think of Hund's rule as "bus seat" behavior. People take an empty seat of their own first and only sit together when there are no empty seats left. If you see pairing in an orbital set while other orbitals in the same subshell are empty, the answer is Hund's rule.
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44.
The hardest form of carbon is
1. A.charcoal
2. B.coke
3. C.diamond
4. D.graphite
Show answer
Answer: C. diamond
AI explanation
1. Analyze the question The question asks which allotrope or form of carbon is the hardest. Carbon exists in several forms, and their hardness depends on how the atoms are bonded.
2. Explain the concept Carbon has crystalline allotropes (diamond, graphite) and amorphous forms (charcoal, coke, lampblack).
Diamond: Each carbon atom is covalently bonded to four others in a rigid, three-dimensional tetrahedral network. This strong, uniform bonding makes it the hardest known natural substance.
Graphite: Carbon atoms are arranged in flat hexagonal layers. The layers are held together only by weak van der Waals forces, so they slide over each other. This makes graphite soft and slippery, which is why it is used in pencils and as a lubricant.
Charcoal and coke: These are amorphous (non-crystalline) forms with no ordered structure. They are brittle and soft, and are used as fuels and as reducing agents.
3. Solution
A. Charcoal: amorphous and soft, so not the hardest.
B. Coke: amorphous and brittle, so not the hardest.
C. Diamond: has a giant tetrahedral covalent structure and is the hardest.
D. Graphite: has a layered structure and is soft.
4. Answer The hardest form of carbon is diamond, which is option C.
5. Exam tip Remember: "Diamond = 3D, Graphite = layers." Strong bonds in all three dimensions give hardness, while layers that slide give softness. A related point that often appears in exams is that graphite conducts electricity because of its free (delocalized) electrons, while diamond does not.
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45.
The oxidation number of Chromium in Na $_2$ Cr $_2$ O $_7$ is
1. A.+2
2. B.+6
3. C.+7
4. D.+12
Show answer
Answer: B. +6
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46.
Naturally occurring Boron is made up of 19.9% $^{10}$ B and 80.1% $^{11}$ B. The relative atomic mass of Boron is
1. A.21.0
2. B.10.8
3. C.10.5
4. D.10.0
Show answer
Answer: B. 10.8
AI explanation
Analyze the question: We are given the percentage abundance of two isotopes of boron and asked to find the relative atomic mass (average atomic mass).
Concept: Relative atomic mass is the weighted average of the isotopic masses:
RAM = (mass₁ × %₁ + mass₂ × %₂) / 100
Solution:
Contribution of ¹⁰B: 10 × 19.9 = 199
Contribution of ¹¹B: 11 × 80.1 = 881.1
Sum = 199 + 881.1 = 1080.1
RAM = 1080.1 / 100 = 10.801 ≈ 10.8
Check: The answer must lie between 10 and 11. Since ¹¹B is far more abundant (about 80%), the answer should be much closer to 11, which fits 10.8.
Tip: A quick shortcut is to start at the lighter isotope and add the heavy isotope's fraction: 10 + 0.801 × 1 = 10.801. Also, you can eliminate options A (21.0) and D (10.0) immediately, because the average must fall between the two isotopic masses and be nearer the more abundant one.
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47.
Which of the following statements about ammonium salt is correct? It
1. A.dissolves in water to form solution of pH > 1
2. B.dissolves in water to form solution of pH < 7
3. C.does not decompose on heating
4. D.is insoluble in water
Show answer
Answer: B. dissolves in water to form solution of pH < 7
AI explanation
Analyze the question: We need the true statement about ammonium salts (e.g., NH₄Cl, (NH₄)₂SO₄, NH₄NO₃).
Concept: Ammonium salts are soluble in water. The ammonium ion (NH₄⁺) is the conjugate acid of a weak base (NH₃), so it undergoes hydrolysis:
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
This produces H₃O⁺ ions, making the solution slightly acidic, with a pH below 7 (for salts of strong acids such as NH₄Cl).
Check each option:
A. pH > 1: This is technically true for a typical ammonium salt solution (for example, pH is about 5 for NH₄Cl), but it is a vague and misleading statement. It would also be true of pure water and of alkaline solutions, so it does not describe the behavior of ammonium salts. It is not the best answer.
B. pH < 7: Correct. The hydrolysis of NH₄⁺ gives an acidic solution.
C. Does not decompose on heating: False. Ammonium salts decompose on heating. For example, NH₄Cl → NH₃ + HCl (it sublimes and dissociates), and NH₄NO₃ decomposes to N₂O and water.
D. Insoluble in water: False. All common ammonium salts are soluble.
Answer: B.
Tip: Remember "salt of a strong acid and a weak base gives an acidic solution." Ammonium salts also all dissolve in water and all decompose on heating, and these two facts are often tested along with the pH.
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48.
In ethanol, the attractive forces between adjacent molecules are
1. A.covalent bonds only
2. B.hydrogen bonds only
3. C.hydrogen bonding and Van der Waal's forces
4. D.Van der Waal's forces only
Show answer
Answer: C. hydrogen bonding and Van der Waal's forces
AI explanation
Step 1: Analyze the question The question asks about the forces between adjacent ethanol molecules (intermolecular forces), not the bonds within a molecule.
Step 2: Explain the concept
Covalent bonds hold atoms together inside a molecule (intramolecular). They are not the attractive forces between molecules, so option A is out.
Van der Waals (London dispersion) forces exist between all molecules. They come from temporary and induced dipoles.
Hydrogen bonding occurs when hydrogen is bonded directly to a highly electronegative atom (N, O or F), and is attracted to a lone pair on a neighbouring molecule's N, O or F atom.
Step 3: Apply to ethanol Ethanol is CH₃CH₂OH.
It has an -OH group, so hydrogen bonding occurs between the H of one molecule and the O of another.
It also has an ethyl group (CH₃CH₂–), a non-polar hydrocarbon part. The molecules therefore also experience Van der Waals (dispersion) forces, which exist in every molecule.
So ethanol molecules attract each other through both hydrogen bonding and Van der Waals forces.
Step 4: Eliminate the options
A. Wrong: covalent bonds are intramolecular.
B. Wrong: "only" ignores the Van der Waals forces that are always present.
C. Correct: both forces are present.
D. Wrong: it ignores the hydrogen bonding from the -OH group.
Tip: Van der Waals forces are present in all molecules. If a molecule also has O-H, N-H or F-H, add hydrogen bonding. Watch for the word "only" in options, as it is often a trap. This also explains why ethanol (boiling point about 78 °C) boils much higher than dimethyl ether, which has no hydrogen bonding.
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49.
Pure water contaminated with quicklime will have a pH of
1. A.1
2. B.6
3. C.7
4. D.8
Show answer
Answer: D. 8
AI explanation
Analyze the question Quicklime is calcium oxide (CaO). The question asks what pH pure water will have once it is contaminated with it.
Explain the concept
Quicklime is a basic oxide. When it dissolves in water, it forms calcium hydroxide (slaked lime), which is an alkali:
CaO + H₂O → Ca(OH)₂
Ca(OH)₂ ionizes to give hydroxide ions:
Ca(OH)₂ → Ca²⁺ + 2OH⁻
Extra OH⁻ ions make the solution basic, so the pH is greater than 7.
Step-by-step solution
Pure water has a pH of 7 (neutral).
Adding CaO produces Ca(OH)₂, which releases OH⁻ ions.
The solution becomes alkaline, so the pH rises above 7.
Checking the options: • A (1): strongly acidic, so wrong. • B (6): weakly acidic, so wrong. • C (7): neutral, so wrong. • D (8): slightly alkaline, which fits.
Note on the value: In reality, a saturated calcium hydroxide solution has a pH of about 12 to 12.5. But among the options given, only D is above 7, so D is the only one that fits an alkaline solution.
Answer: D (8)
Exam tip: Remember that metal oxides are basic and non-metal oxides are acidic. For example, CaO, MgO and Na₂O give alkaline solutions, while CO₂, SO₂ and P₂O₅ give acidic ones. When a question asks about the pH after adding a metal oxide, pick the option above 7.
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50.
Which of the following is a monomer of polythene?
1. A.Ethanol
2. B.Vinyl chloride
3. C.Ethene
4. D.Ethane
Show answer
Answer: C. Ethene
AI explanation
Step 1: Analyze the question We need the monomer (the small repeating unit) that joins together to form polythene (polyethylene).
Step 2: Explain the concept Polythene is made by addition polymerization. In this process, monomers containing a carbon-carbon double bond (unsaturated molecules) open up their double bonds and join end to end to form a long chain.
Step 3: Work through the options
A. Ethanol (C₂H₅OH): An alcohol with no C=C double bond. It is not used to make polythene.
B. Vinyl chloride (CH₂=CHCl): It has a double bond, but it is the monomer of PVC (polyvinyl chloride), not polythene.
C. Ethene (CH₂=CH₂): An unsaturated alkene with a double bond. This is the monomer of polythene.
D. Ethane (C₂H₆): A saturated alkane with only single bonds. It cannot undergo addition polymerization.
Step 4: The reaction
n CH₂=CH₂ → –(CH₂–CH₂)ₙ–
(ethene, under high pressure and a catalyst, forms polythene, also called polyethylene)
Answer: The monomer of polythene is ethene, which is option C.
Exam tip: The name tells you the monomer: poly + ethene = polyethene (polythene). Likewise, poly + vinyl chloride = PVC, and poly + propene = polypropene. Always look for a double bond when the question asks about addition polymer monomers.
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2024 WAEC (WASSCE) Chemistry Past Questions and Answers

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