Stoichiometry and the mole

📌 Key points

1. Relative formula mass (Mr) is the sum of the relative atomic masses of all the atoms in the formula.
2. One mole of a substance contains 6.02 × 10²³ particles (the Avogadro constant).
3. Number of moles = mass (g) ÷ molar mass (g/mol), where molar mass is Mr in grams.
4. One mole of any gas occupies 24 dm³ at room temperature and pressure (r.t.p.), so moles of gas = volume (dm³) ÷ 24.
5. Concentration (mol/dm³) = moles ÷ volume (dm³). To change cm³ to dm³, divide by 1000.
6. A balanced equation gives the mole ratio of reactants and products, which is used to calculate reacting masses and volumes.
7. The empirical formula is the simplest whole-number ratio of atoms; the molecular formula is the actual number of each type of atom in a molecule.
8. Percentage yield = actual yield ÷ theoretical yield × 100; percentage purity = mass of pure product ÷ mass of impure sample × 100.
9. State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).

📖 Key terms

Mole

The amount of substance containing 6.02 × 10²³ particles.

Relative formula mass (Mr)

Sum of the relative atomic masses of the atoms in a formula.

Empirical formula

The simplest whole-number ratio of the atoms of each element in a compound.

Molecular formula

The number and type of each atom in one molecule.

Limiting reactant

The reactant that is completely used up and so limits the amount of product.

Concentration

The amount of solute in a given volume of solution, in mol/dm³ or g/dm³.