IGCSE · Practice questions · Relative masses

Weigh it against carbon.

Six original Cambridge-style questions on the carbon-12 scale: the definition of A_r, relative formula masses, percentage composition and the average mass of isotopes.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Everything is compared with carbon-12.

Standard: 1/12 of the mass of an atom of carbon-12.
M_r: add the A_r of every atom; a number after a bracket multiplies everything inside.
% by mass = (A_r × number of atoms) ÷ M_r × 100.
No units: relative masses are ratios.

Recall

[2 marks]

Define relative atomic mass, A_r.

The average mass of the isotopes of an element, ✓
compared with 1/12 of the mass of an atom of carbon-12. ✓

Calculation

[2 marks]

Calculate the relative formula masses of (a) calcium hydroxide, Ca(OH)₂, and (b) magnesium nitrate, Mg(NO₃)₂. (A_r: H 1, N 14, O 16, Mg 24, Ca 40)

(a) 40 + 2 × (16 + 1) = 40 + 34 = 74. ✓
(b) 24 + 2 × (14 + 3 × 16) = 24 + 2 × 62 = 148. ✓

Calculation

[2 marks]

Calculate the relative formula mass of hydrated copper(II) sulfate, CuSO₄·5H₂O. (A_r: H 1, O 16, S 32, Cu 64)

CuSO₄: 64 + 32 + 4 × 16 = 160. ✓
Add 5 × 18 = 90 for the water of crystallisation: 160 + 90 = 250. ✓

Calculation

[3 marks]

Iron is extracted from iron(III) oxide, Fe₂O₃. Calculate the percentage by mass of iron in Fe₂O₃, and the maximum mass of iron that could be obtained from 400 tonnes of it. (A_r: O 16, Fe 56)

M_r = 2 × 56 + 3 × 16 = 160. ✓
% Fe = 112 ÷ 160 × 100 = 70%. ✓
Mass of iron = 70% of 400 tonnes = 0.70 × 400 = 280 tonnes. ✓

Analysis

[3 marks]

A farmer can buy urea, CO(NH₂)₂, or ammonium sulfate, (NH₄)₂SO₄. Use percentages by mass of nitrogen to decide which supplies more nitrogen per kilogram. (A_r: H 1, C 12, N 14, O 16, S 32)

Urea: M_r = 12 + 16 + 2 × 16 = 60; % N = 28 ÷ 60 × 100 = 46.7%. ✓
Ammonium sulfate: M_r = 132; % N = 28 ÷ 132 × 100 = 21.2%. ✓
Urea supplies more nitrogen per kilogram, more than twice as much. ✓

Calculation

[2 marks]

Extended. Chlorine consists of 75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass of chlorine.

Total mass of 100 atoms = 35 × 75 + 37 × 25 = 2625 + 925 = 3550. ✓
A_r = 3550 ÷ 100 = 35.5. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.