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2024 O Level Chem P1 Q18 — Preparation of Salts Solution

Preparation of Salts 2024 O Level Pure Chemistry · Paper 1 · Q18

Question

Four pairs of solutions are listed.
1 NH₄Cl and NaOH.
2 LiCl and AgNO₃.
3 CH₃COOH and KOH.
4 CaCl₂ and Ba(NO₃)₂. Which row identifies the pair that can be mixed together to produce a solid salt by precipitation and the pair that can produce a salt by titration followed by crystallisation?

Aprecipitation: 1, titration followed by crystallisation: 3
Bprecipitation: 1, titration followed by crystallisation: 4
Cprecipitation: 2, titration followed by crystallisation: 3
Dprecipitation: 2, titration followed by crystallisation: 4

Correct answer: C

Video Solution

Explanation

Answer: C. Two methods, two pairs.

Precipitation requires both reactants to be solutions and the product to be an insoluble salt. Pair 2 fits: \(\text{LiCl} + \text{AgNO}_3 \rightarrow \text{AgCl(s)} + \text{LiNO}_3(\text{aq})\). Silver chloride is insoluble. Pair 4 doesn't precipitate — \(\text{BaCl}_2\) and \(\text{Ca(NO}_3)_2\) are both soluble.

Titration followed by crystallisation applies to soluble salts made from acid + alkali. Pair 3 fits: \(\text{CH}_3\text{COOH} + \text{KOH} \rightarrow \text{CH}_3\text{COOK} + \text{H}_2\text{O}\). Pair 1 (\(\text{NH}_4\text{Cl} + \text{NaOH}\)) produces ammonia gas instead of staying in solution, so titration is impractical.

Precipitation = 2; titration = 3 → row C.

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