The Mole and Molar Mass — AQA GCSE Chemistry (Higher Tier)
The mole lets chemists count particles by weighing. It is central to quantitative chemistry.
What is a mole?
Chemical amounts are measured in moles (mol). One mole of any substance contains the same number of particles — the Avogadro constant, 6.02 × 10²³ particles.
Molar mass
The mass of one mole of a substance (its molar mass) in grams is equal to its relative formula mass (Mr).
For example:
- 1 mole of water (Mr 18) has a mass of 18 g.
- 1 mole of carbon dioxide (Mr 44) has a mass of 44 g.
The key equation
$$\text{number of moles} = \frac{\text{mass (g)}}{\text{molar mass (Mr)}}$$
Rearrange to find mass (mass = moles × Mr) or molar mass.
Worked examples
- How many moles in 36 g of water? 36 ÷ 18 = 2 mol.
- What is the mass of 0.5 mol of CO₂? 0.5 × 44 = 22 g.
- How many molecules in 2 mol? 2 × 6.02 × 10²³ = 1.2 × 10²⁴.
Exam tips
- One mole contains 6.02 × 10²³ particles (Avogadro constant).
- Molar mass (g) = relative formula mass.
- Learn moles = mass ÷ Mr and practise rearranging it.
- Show your working and watch units (grams).