Gas volumes
Molar gas volume (Higher tier only)
At — 20 °C and 1 atmosphere — takes up (24 000 cm³). It doesn't matter which gas it is.
volume of gas (dm³) = moles × 24
moles = volume (dm³) ÷ 24
Example 1: what volume does 0.25 mol of oxygen take up at RTP?
volume = 0.25 × 24 = 6.0 dm³
Example 2: how many moles are in 120 cm³ of hydrogen at RTP?
moles = 120 ÷ 24 000 = 0.005 mol
Key terms
Volumes from masses (Higher tier only)
Combine moles and molar volume to go from a mass to a volume.
Example: what volume of carbon dioxide is made when 4.4 g of CO₂ forms (Mᵣ 44)?
- moles = 4.4 ÷ 44 = 0.1 mol
- volume = 0.1 × 24 =
In a reaction, use the equation's mole ratio first. For example, in 2Na + 2H₂O → 2NaOH + H₂, 0.2 mol of sodium makes 0.1 mol of hydrogen, which is 0.1 × 24 = 2.4 dm³.
Key terms
Avogadro's law (Higher tier only)
at the same temperature and pressure, .
This means the volumes of gases in a reaction are in the in the equation.
Example: H₂(g) + Cl₂(g) → 2HCl(g)
The ratio is 1 : 1 : 2, so 50 cm³ of hydrogen reacts with 50 cm³ of chlorine to make 100 cm³ of hydrogen chloride.