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Gas VolumesOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is the molar gas volume?

At room temperature and pressure (RTP), one mole of any gas occupies the same volume, regardless of what the gas is.

This volume is assumed to be 24 dm³ (24,000 cm³) per mole at RTP.

This is true for any gas — hydrogen, oxygen, carbon dioxide, etc. — because it is the number of particles (not their identity) that determines the volume a gas takes up.

Key termsmolar gas volumeroom temperature and pressure (RTP)

Section 2

How do you calculate the volume of a gas from its moles?

Use the equation:

V = n × 24 dm³

where V is the volume in dm³ and n is the number of moles of gas.

To convert dm³ to cm³, multiply by 1000 (since 1 dm³ = 1000 cm³).

Example

Calculate the volume occupied by 0.5 mol of oxygen gas at RTP. V = 0.5 × 24 = 12 dm³.

Section 3

How do you calculate the number of moles from a gas volume?

Rearranging the equation gives:

n = V ÷ 24

Remember to convert the volume into dm³ first if it is given in cm³ (divide by 1000).

Example

A reaction produces 6000 cm³ of carbon dioxide at RTP. Convert to dm³: 6000 ÷ 1000 = 6 dm³. Moles = 6 ÷ 24 = 0.25 mol.

Common mistake

A very common error is forgetting to convert cm³ to dm³ before dividing by 24 — always check the units given in the question first.

Section 4

How is gas volume used in reacting quantities calculations?

Gas volume calculations are often combined with balanced symbol equations to find the volume of a gas produced or used up in a reaction.

  1. Work out the number of moles of the known substance
  2. Use the mole ratio from the balanced equation to find the moles of the gas
  3. Convert moles of gas to volume using V = n × 24
Example

Magnesium reacts with excess hydrochloric acid: Mg + 2HCl → MgCl2 + H2. Find the volume of hydrogen gas produced from 0.1 mol of magnesium. Mole ratio Mg:H2 is 1:1, so moles of H2 = 0.1. Volume = 0.1 × 24 = 2.4 dm³.

Must Know

  • One mole of any gas occupies 24 dm³ at room temperature and pressure (RTP)
  • V = n × 24 dm³, and rearranged: n = V ÷ 24
  • 1 dm³ = 1000 cm³ — always convert cm³ to dm³ before using the equation
  • The identity of the gas does not affect the molar volume — only the number of moles does
  • Gas volume calculations combine with balanced equations and mole ratios to find reacting volumes

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