Gas volumes · Year 12
At the same temperature and pressure, one mole of any gas fills the same volume. At room temperature and pressure (about 20°C and 1 atmosphere) this molar volume is 24 dm³; at STP (0°C and 1 atmosphere) it is 22.4 dm³. So you can turn volumes of gases into moles, and use balanced equations to find the volumes of gases that react or form.
Key facts
- One mole of any gas fills 24 dm³ (24,000 cm³) at room temperature and pressure (RTP).
- At STP (0°C and 1 atmosphere) one mole of gas fills 22.4 dm³: CAPS and US courses use this value.
- volume of gas = moles × molar volume, so moles = volume ÷ molar volume.
- Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (Avogadro’s law).
- So for gases, the ratio of volumes is the same as the mole ratio in the balanced equation.
- Change cm³ into dm³ by dividing by 1000.