Volumes of Gases and Molar Volume — AQA GCSE Chemistry (Higher / Separate)
Equal volumes of gases contain equal numbers of moles, which lets us calculate gas volumes in reactions.
Molar gas volume
At room temperature and pressure (rtp) — about 20 °C and normal atmospheric pressure — one mole of any gas occupies 24 dm³ (24 000 cm³). This is the molar volume.
$$\text{volume of gas (dm}^3) = \text{moles} \times 24$$
Rearranged: moles = volume ÷ 24.
Worked examples
- What volume does 2 mol of CO₂ occupy at rtp? 2 × 24 = 48 dm³.
- How many moles in 12 dm³ of gas? 12 ÷ 24 = 0.5 mol.
Using gas volumes in reactions
Combine the molar gas volume with the mole ratio from the balanced equation:
- Find the moles of the known substance.
- Use the equation ratio to find moles of the gas.
- Multiply by 24 to get the volume in dm³.
Avogadro's idea
Because equal volumes of gases (at the same temperature and pressure) contain equal numbers of molecules, the ratio of gas volumes in a reaction equals the ratio of moles in the balanced equation.
Exam tips
- Learn that 1 mole of gas = 24 dm³ at rtp.
- Use volume = moles × 24 (or moles = volume ÷ 24).
- Combine with the equation ratio to find gas volumes in reactions.
- Keep volumes in dm³ (or convert cm³ ÷ 1000).