The mole and reacting masses — GCSE Chemistry practice questions

Question 1

Iron tablets for people who are short of iron contain hydrated iron(II) sulfate. Its formula is FeSO₄·7H₂O.

What is the relative formula mass, Mr, of FeSO₄·7H₂O?

Accepted Answer

D: 278. Mr of FeSO₄ = 56 + 32 + 4 × 16 = 152. Seven water molecules add 7 × 18 = 126, so Mr = 152 + 126 = 278.

Question 2

A colourless liquid used as a solvent in nail-varnish remover contains carbon, hydrogen and oxygen only.

| element | carbon | hydrogen | oxygen |
|---|---|---|---|
| percentage by mass | 62.07 | 10.34 | 27.59 |

What is the empirical formula of the liquid?

Accepted Answer

C: C₃H₆O. In 100 g: C 62.07 ÷ 12 = 5.17 mol, H 10.34 ÷ 1 = 10.34 mol, O 27.59 ÷ 16 = 1.72 mol. Dividing by 1.72 gives 3 : 6 : 1, so the empirical formula is C₃H₆O.

Question 3

A geologist heats 5.00 g of a powdered ore. The only substance in the ore that decomposes is copper(II) carbonate.

CuCO₃ → CuO + CO₂

The ore gives 0.720 dm³ of carbon dioxide at r.t.p.

One mole of any gas occupies 24 dm³ at r.t.p.

What is the percentage by mass of copper(II) carbonate in the ore?

Accepted Answer

C: 74.4 %. Moles of CO₂ = 0.720 ÷ 24 = 0.0300 mol = moles of CuCO₃. Mass = 0.0300 × 124 = 3.72 g, and 3.72 ÷ 5.00 × 100 = 74.4 %.