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2020 O Level Chem P1 Q21 — Salts / Preparation of Salts Solution

Salts / Preparation of Salts 2020 O Level Pure Chemistry · Paper 1 · Q21

Question

A salt can be prepared by adding an excess of solid X to an aqueous solution of Y. A solution and some solid X remain. The mixture is filtered. The filtrate is evaporated to form a concentrated solution, which is left to crystallise. Excess solution is poured off, leaving salt Z. Which salt can be safely prepared using this method?

Aammonium chloride
Bbarium sulfate
Ccopper(II) sulfate
Dpotassium chloride

Correct answer: C

Video Solution

Explanation

Answer: C (copper(II) sulfate). The method described — adding an excess of an insoluble solid to an acid, filtering off the unreacted solid, then crystallising the filtrate — is the standard route for making a soluble salt of a metal whose oxide or carbonate is insoluble in water. The excess solid guarantees all the acid is used up, and the leftover solid is simply filtered away.

Copper(II) sulfate fits perfectly: insoluble copper(II) oxide reacts with dilute sulfuric acid to give copper(II) sulfate solution, which is then crystallised. So C is correct.

Ammonium chloride (A) and potassium chloride (D) come from two soluble reactants, so they are made by titration, not by this excess-solid method. Barium sulfate (B) is insoluble in water, so it is made by precipitation, not by crystallising a filtrate. The answer is C.

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