What is a mole?
Atoms and molecules are far too small to count one by one, so chemists count in moles. One mole of any substance contains the same number of particles as there are atoms in exactly 12 g of carbon-12. This number is the Avogadro constant, 6.02 ×1023 per mole.
Key idea
Number of moles n = mass (g) ÷ molar mass (g/mol), i.e. n = m ÷ M.
Number of particles = n × 6.02 ×1023.
For a gas at r.t.p.: volume = n × 24 dm³.
Molar mass
The molar mass M (in g/mol) is numerically equal to the relative formula mass (Mr). For water H2O, Mr = (2×1) + 16 = 18, so M = 18 g/mol.
Worked example
How many moles are in 88 g of carbon dioxide, CO2? First find Mr: 12 + (2×16) = 44 g/mol.
n = m ÷ M = 88 ÷ 44 = 2 mol.
Number of molecules = 2 × 6.02 ×1023 = 1.204 ×1024 molecules.
Remember
- r.t.p. = room temperature and pressure; 1 mol of any gas occupies 24 dm³ (24 000 cm³).
- Molar mass has units g/mol; the Avogadro constant has units per mol.