GCSE Chemistry · AQA 8462 · Topic 4.3.5

Use of amount of substance in relation to volumes of gases

Key points

(Higher) Equal amounts in moles of any gases occupy the same volume under the same conditions of temperature and pressure.
At room temperature and pressure (RTP, 20 °C and 1 atmosphere) one mole of any gas occupies 24 dm³ (24 000 cm³).
$\text{volume of gas (dm}^3) = \text{moles} \times 24$ and $\text{moles} = \frac{\text{volume (dm}^3)}{24}$ .
Because volume is proportional to moles, the ratio of gas volumes in a reaction equals the mole ratio in the balanced equation, e.g. $2H_2 + O_2 \rightarrow 2H_2O$ means 20 cm³ of hydrogen reacts with 10 cm³ of oxygen.
You can combine this with $\text{moles} = \frac{\text{mass}}{M_r}$ to find the volume of gas produced from a given mass of reactant.

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Gases are easier to measure by volume than by mass. Because equal numbers of moles of any gas take up the same volume under the same conditions, gas volumes can be calculated directly from moles. This sub-topic is (Higher tier) and…

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