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2025 O Level Chem P1 Q16 — Salts / Acids and Bases Solution

Salts / Acids and Bases 2025 O Level Pure Chemistry · Paper 1 · Q16

Question

When dilute sulfuric acid and aqueous sodium hydroxide react, sodium hydrogen sulfate, NaHSO₄, is first formed. H₂SO₄(aq) + NaOH(aq) → NaHSO₄(aq) + H₂O(l). Aqueous sodium hydrogen sulfate dissociates into ions. NaHSO₄(aq) → Na⁺(aq) + H⁺(aq) + SO₄²⁻(aq). A different product, sodium sulfate, Na₂SO₄, will form if more sodium hydroxide is added. H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l). Which results for these two salts, when tested with Universal Indicator and barium nitrate solution, are correct? Question diagram

Question diagram

A
B
C
D

Correct answer: D

Video Solution

Explanation

Answer: D. Test each salt:

NaHSO₄: the dissociation equation shows it produces \(\text{H}^{+}\) in solution — so it is acidic, pH < 7. The \(\text{SO}_4^{2-}\) anion reacts with aqueous barium nitrate \(\text{Ba(NO}_3)_2\) to give a white precipitate of barium sulfate \(\text{BaSO}_4\).

Na₂SO₄: sodium sulfate is a neutral salt (strong base + strong acid), pH = 7. The \(\text{SO}_4^{2-}\) anion again gives a white precipitate of \(\text{BaSO}_4\) with barium nitrate.

Both salts produce a white precipitate with aqueous barium nitrate (the standard test for sulfate ions). Only D combines the correct pH for NaHSO₄ (< 7) with the white precipitates from both salts.

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