Preparation of salts — GCSE Chemistry practice questions

Question 1

A student shakes a small amount of each solid below with water in a test tube.

Which solid does not dissolve?

Accepted Answer

A: barium sulfate. All nitrates and all potassium salts are soluble, so copper(II) nitrate and potassium carbonate dissolve. Sulfates are soluble except barium, calcium and lead(II) sulfates, so magnesium sulfate dissolves but barium sulfate does not.

Question 2

Silver chloride is used in some photography demonstrations. A technician makes pure, dry silver chloride by mixing aqueous silver nitrate with dilute hydrochloric acid.

Which process is not needed?

Accepted Answer

A: crystallisation. Silver chloride is insoluble, so it forms as a precipitate. It is filtered off, washed with distilled water to remove the soluble nitric acid and any excess reactant, then dried. There is no salt solution to crystallise.

Question 3

A zinc supplement factory checks a batch of hydrated zinc sulfate crystals. Their formula is ZnSO₄· xH₂O.

A 2.87 g sample of the crystals is heated until its mass stops changing. The anhydrous zinc sulfate left has a mass of 1.61 g.

What is the value of x?

Accepted Answer

C: 7. Mass of water = 2.87 − 1.61 = 1.26 g, which is 1.26 ÷ 18 = 0.0700 mol. Moles of zinc sulfate = 1.61 ÷ 161 = 0.0100 mol. The ratio is 0.0100 : 0.0700 = 1 : 7, so x = 7.