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2021 O Level Chem P1 Q18 — Preparation of Salts Solution

Preparation of Salts 2021 O Level Pure Chemistry · Paper 1 · Q18

Question

Salt P can be prepared by reacting together either a dilute acid and a metal, or an acid and an insoluble carbonate. A pure sample of salt P is then obtained by filtration, followed by crystallisation of the filtrate. Which salt is P?

Acopper(II) sulfate
Blead(II) sulfate
Cmagnesium chloride
Dsodium chloride

Correct answer: C

Video Solution

Explanation

Answer: C (magnesium chloride). Salt P must be preparable by both routes. The acid + metal route needs a metal above hydrogen in the reactivity series but not dangerously reactive. The acid + insoluble carbonate route needs an insoluble carbonate (so excess can be filtered off) that gives a soluble salt able to crystallise from the filtrate.

Magnesium fits both: Mg + 2HCl → MgCl₂ + H₂, and MgCO₃ (insoluble) + 2HCl → MgCl₂ + H₂O + CO₂; magnesium chloride is soluble.

Copper is below hydrogen, so it does not react with dilute acid. Lead(II) sulfate is insoluble, so it cannot crystallise from solution. Sodium is far too reactive for the metal + acid method, and sodium carbonate is soluble, not insoluble.

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