NECO 2001 Chemistry · Question 33

To test for the presence of chloride ions in ion, correct sequence of reagents to use is

Options

A.Ba(NO3)2(aq) then HNO3(aq).
B.HCl(aq) then Ba(NO3)2(aq).
C.HCl(aq) then Ag3(aq).
D.HNO3(aq) then fresh FeSO4(aq).
E.HNO3(aq) then AgNO3(aq).

Answer: E. HNO3(aq) then AgNO3(aq).

Explanation written with AI and checked against the answer key

Analyze the question: The question asks for the correct sequence of reagents for testing chloride ions (Cl⁻) in a solution. The wording "in ion" is a typo for "in solution."

Concept: The standard test for chloride ions uses silver nitrate solution, but it must be done in the right order:

Add dilute nitric acid first. This removes interfering ions such as carbonate (CO₃²⁻) and sulphite (SO₃²⁻), which would also form precipitates with silver ions (Ag₂CO₃, Ag₂SO₃).
Then add silver nitrate solution (AgNO₃). If chloride ions are present, a white precipitate of silver chloride forms.

Ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white precipitate)

The precipitate dissolves in dilute ammonia solution, which confirms it is AgCl.

Checking the options:

A. Ba(NO₃)₂ then HNO₃ is the test for sulphate ions (white BaSO₄ that is insoluble in acid), not chloride.
B. HCl then Ba(NO₃)₂ is also a sulphate test, and adding HCl would introduce chloride ions, which defeats the purpose.
C. HCl would add chloride ions of its own, giving a false positive. "Ag₃(aq)" is also not a valid reagent (it should be AgNO₃).
D. HNO₃ then fresh FeSO₄ is part of the brown ring test for nitrate ions.
E. HNO₃ then AgNO₃ is the correct chloride test.

Answer: E

Tip: Remember the pairing:

Chloride: HNO₃, then AgNO₃ (white precipitate)
Sulphate: HCl (or HNO₃), then BaCl₂ or Ba(NO₃)₂ (white precipitate)
Nitrate: FeSO₄ and concentrated H₂SO₄ (brown ring)

Never acidify with HCl when testing for chloride, and never with H₂SO₄ when testing for sulphate.

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