Higher Tier only. This is an optional extension after the core lessons. Go to exam practice.
Higher Tier only. The unit for amount of substance is the mole, mol. One mole contains 6.02 × 10²³ of the stated particles. This is the Avogadro constant per mole. Name the particles: carbon atoms, water molecules, sodium ions, and so on.
One mole of carbon atoms has a mass of 12 g. One mole of H₂O molecules has a mass of 18 g. Both contain the same number of their stated particles.
The molar mass in g/mol is numerically equal to Ar or Mr. Mr itself still has no unit.
Amount (mol) = mass (g) ÷ molar mass (g/mol).
Often written n = m/Mr in GCSE working, using the numerically equal molar mass.
9 g water: 9 ÷ 18 = 0.50 mol.
Mass = amount × molar mass.
0.25 mol CO₂, Mr = 44:
0.25 × 44 = 11 g.
Use grams: 500 mg = 0.500 g; 1 kg = 1000 g.
Number of particles = amount in mol × 6.02 × 10²³.
0.50 mol water contains 3.01 × 10²³ water molecules. Each molecule has three atoms, so the total number of atoms is three times its number of molecules.
n = mass ÷ molar mass. Mass = n × molar mass. One mole contains 6.02 × 10²³ stated particles.
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