Volumes of Gases

This lesson covers how to calculate the volume of a gas produced or consumed in a chemical reaction, using the concept of molar gas volume, as required by the Edexcel GCSE Combined Science specification (1SC0). This topic is primarily assessed at Higher tier.

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Molar Gas Volume

At room temperature and pressure (RTP) — defined as approximately 20 °C and 1 atmosphere (1 atm) — one mole of any gas occupies a volume of:

$\text{molar gas volume} = 24 \text{ dm}^3 = 24{,}000 \text{ cm}^3$molar gas volume=24 dm3=24,000 cm3

This applies to all gases, regardless of their identity, because at the same temperature and pressure, gas particles are spaced equally far apart (Avogadro's law).

Exam Tip: You will be given the value of 24 dm³ in the exam — you do not need to memorise it. But you do need to know how to use it.

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The Key Equation

$\text{volume (dm}^3) = \text{moles} \times 24$volume (dm3)=moles×24

Or equivalently:

$\text{moles} = \frac{\text{volume (dm}^3)}{24}$moles=24volume (dm3)​

If working in cm³:

$\text{volume (cm}^3) = \text{moles} \times 24{,}000$volume (cm3)=moles×24,000

$\text{moles} = \frac{\text{volume (cm}^3)}{24{,}000}$moles=24,000volume (cm3)​

The Triangle