Using Moles to Balance Equations and Calculate Reacting Volumes of Gases — AQA GCSE Chemistry (Higher / Separate)
Moles can be used to deduce balanced equations from experimental data and to calculate the volumes of gases that react.
Balancing equations using moles
If you know the masses of reactants and products, you can find a balanced equation:
- Convert each mass to moles (moles = mass ÷ Mr).
- Find the simplest whole-number ratio of the moles.
- Use this ratio as the balancing numbers in the equation.
Example
If 0.2 mol of a metal reacts with 0.1 mol of oxygen, the ratio metal : O₂ is 2 : 1, giving 2M + O₂ → 2MO.
Reacting volumes of gases
For reactions involving gases, the ratio of the volumes of gases equals the ratio of moles in the balanced equation (at the same temperature and pressure). So you can use the equation directly to find gas volumes.
Example
N₂ + 3H₂ → 2NH₃: 1 volume of nitrogen reacts with 3 volumes of hydrogen to make 2 volumes of ammonia. So 10 cm³ of N₂ needs 30 cm³ of H₂.
Exam tips
- Convert masses to moles, then find the simplest ratio to balance an equation.
- For gases, the volume ratio = mole ratio (same conditions).
- Combine with the molar gas volume (24 dm³) when masses/volumes are mixed.
- Show working clearly and check the equation balances.