NECO 2009 Chemistry · Question 9

Which of the following equations is NOT balanced?

Options

A.CH4(g) + 202(9) → CO2(g) + 2H2O(g)
B.CU(S) + HNO3(aq) → Cu(NO3)2(aq) + 4H2O(I) + NO(g)
C.2KCIO3(s) → 2KCI(s)+ 3O2(g)
D.NaH(s) + H2O(1) → NaOH(aq) +H2(g)
E.Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

Answer: B. CU(S) + HNO3(aq) → Cu(NO3)2(aq) + 4H2O(I) + NO(g)

Explanation written with AI and checked against the answer key

Concept: An equation is balanced when each element has the same number of atoms on both sides. Count atoms for each option.

A. CH4 + 2O2 → CO2 + 2H2O

Left: C = 1, H = 4, O = 4
Right: C = 1, H = 4, O = 2 + 2 = 4
Balanced.

B. Cu + HNO3 → Cu(NO3)2 + 4H2O + NO

Left: Cu = 1, H = 1, N = 1, O = 3
Right: Cu = 1, H = 8, N = 2 + 1 = 3, O = 6 + 4 + 1 = 11
The two sides do not match, so this is not balanced. The correctly balanced equation is 3Cu + 8HNO3 → 3Cu(NO3)2 + 4H2O + 2NO.

C. 2KClO3 → 2KCl + 3O2

Left: K = 2, Cl = 2, O = 6
Right: K = 2, Cl = 2, O = 6
Balanced.

D. NaH + H2O → NaOH + H2

Left: Na = 1, H = 1 + 2 = 3, O = 1
Right: Na = 1, O = 1, H = 1 + 2 = 3
Balanced.

E. Na2SO4 + BaCl2 → BaSO4 + 2NaCl

Left: Na = 2, S = 1, O = 4, Ba = 1, Cl = 2
Right: Ba = 1, S = 1, O = 4, Na = 2, Cl = 2
Balanced.

Answer: Option B is the only equation that is not balanced.

Tip: In "NOT balanced" questions, check the equation with the most complicated formulas first, such as those containing brackets like Cu(NO3)2. Also, if a single reactant like HNO3 supplies H, N and O, a quick check on H or N usually exposes the error fast. Here, 1 H on the left against 8 H on the right is an immediate giveaway.

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