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2025 O Level Chem P1 Q19 — Preparation of Salts Solution

Preparation of Salts 2025 O Level Pure Chemistry · Paper 1 · Q19

Question

Barium oxide, BaO, reacts with water to produce barium hydroxide, Ba(OH)₂. Barium hydroxide and barium nitrate, Ba(NO₃)₂, are soluble in water. Barium carbonate, BaCO₃, and barium sulfate, BaSO₄, are insoluble in water. Which method can be used to produce a pure sample of barium sulfate?

AReact dilute sulfuric acid with an excess of BaO(s), then filter to obtain the filtrate and crystallise.
BReact dilute sulfuric acid with an excess of Ba(OH)₂(aq), then leave the solution to crystallise.
CReact dilute sulfuric acid with an excess of Ba(NO₃)₂(aq), then filter to obtain the residue.
DReact dilute sulfuric acid with an excess of BaCO₃(s), then filter to obtain the residue.

Correct answer: C

Video Solution

Explanation

Answer: C. \(\text{BaSO}_4\) is insoluble, so it forms by precipitation. To make a pure sample by precipitation, mix two soluble starting materials so that the unreacted excess and the soluble by-product stay in solution, leaving only \(\text{BaSO}_4\) as the filtered residue.

Option C: \(\text{H}_2\text{SO}_4 + \text{Ba(NO}_3)_2 \rightarrow \text{BaSO}_4 (\text{s}) + 2\text{HNO}_3\). Both starting materials are soluble; the by-product \(\text{HNO}_3\) is soluble. Filter, wash, dry → pure \(\text{BaSO}_4\).

A and D involve insoluble starting materials — barium oxide (\(\text{BaO}\)) and barium carbonate (\(\text{BaCO}_3\)) — which would contaminate the residue. Note that barium oxide is not directly soluble in water as BaO: it reacts with water to give barium hydroxide \(\text{Ba(OH)}_2\), which is only sparingly soluble. B leaves \(\text{BaSO}_4\) mixed in solution with excess barium hydroxide \(\text{Ba(OH)}_2\) — crystallisation can't separate them since \(\text{BaSO}_4\) doesn't dissolve to crystallise.

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