IGCSE · Practice questions · The mole and gas volumes

Count it by mass.

Six original Cambridge-style questions on the mole: mass, number of particles and the molar gas volume of 24 dm³ at r.t.p. Extended content.

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

Go through the mole every time.

moles = mass ÷ M_r with the mass in grams.
particles = moles × 6.02 × 10²³; say which particle: atoms or molecules.
Gas moles = volume in dm³ ÷ 24 (or cm³ ÷ 24 000) at r.t.p.
Gases only for the 24 dm³ rule.

Recall

[1 marks]

State the number of particles in one mole of a substance.

6.02 × 10²³ particles (the Avogadro constant). ✓

Calculation

[3 marks]

For 22 g of carbon dioxide, CO₂, calculate (a) the number of moles, (b) the number of molecules and (c) its volume at r.t.p. (A_r: C 12, O 16)

(a) M_r = 44, so moles = 22 ÷ 44 = 0.5 mol. ✓
(b) 0.5 × 6.02 × 10²³ = 3.01 × 10²³ molecules. ✓
(c) 0.5 × 24 = 12 dm³. ✓

Calculation

[2 marks]

Calculate the volume, in cm³, occupied by 0.16 g of oxygen gas, O₂, at r.t.p. (A_r: O 16)

M_r of O₂ = 32, so moles = 0.16 ÷ 32 = 0.005 mol. ✓
Volume = 0.005 × 24 000 = 120 cm³. ✓

Calculation

[2 marks]

Calculate the mass of 600 cm³ of carbon dioxide at r.t.p. (A_r: C 12, O 16)

Moles = 600 ÷ 24 000 = 0.025 mol. ✓
Mass = 0.025 × 44 = 1.1 g. ✓

Analysis

[3 marks]

A student has 4 g of hydrogen gas, H₂, and 32 g of oxygen gas, O₂. Which sample contains more molecules, and which has the larger volume at r.t.p.? Show your working. (A_r: H 1, O 16)

Hydrogen: 4 ÷ 2 = 2 mol. Oxygen: 32 ÷ 32 = 1 mol. ✓
Hydrogen has twice as many molecules (1.20 × 10²⁴ against 6.02 × 10²³). ✓
Hydrogen also has the larger volume: 2 × 24 = 48 dm³ against 24 dm³. ✓

Calculation

[2 marks]

Calculate the mass of 3.01 × 10²² atoms of zinc. (A_r: Zn 65)

Moles = 3.01 × 10²² ÷ 6.02 × 10²³ = 0.05 mol. ✓
Mass = 0.05 × 65 = 3.25 g. ✓

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.