GCSE Combined Science revision · Chemistry: atoms, bonding and amounts

Higher: moles

Higher only

The idea

A mole of any substance contains 6.02 × 10²³ particles (the Avogadro constant). The mass of one mole is its Mr in grams. Number of moles = mass ÷ Mr. Balanced equations give the ratio of moles reacting, so masses can be calculated from equations, and equations can be balanced from reacting masses.

Find moles: mass ÷ Mr.
Use the equation's ratio.
Convert back: mass = moles × Mr.

Watch out Use the ratio from the balanced equation before converting moles back to mass.

See it worked

Worked example

What mass of water is produced when 4 g of hydrogen burns? 2H₂ + O₂ → 2H₂O (H = 1, O = 16).

Moles of H₂ = 4 ÷ 2 = 2 mol.
Ratio H₂ : H₂O = 2 : 2, so 2 mol of water.
Mass = 2 × 18 = 36 g.

Answer 36 g

Your turn

Question 1Calculator allowed2 marks

How many moles are in 88 g of carbon dioxide (Mr = 44)?

Question 2Calculator allowed2 marks

What mass of magnesium oxide forms from 6 g of magnesium? 2Mg + O₂ → 2MgO (Mg = 24, O = 16).

Question 3Calculator allowed1 mark

How many particles are in one mole of a substance?

In past papers

44 official questions in 28 papers since 2018. The latest:

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