Gas volumes and the ideal gas equation

Molar volume

At room temperature and pressure (r.t.p.) one mole of any gas occupies 24.0 dm³ (24 000 cm³).

n = V / 24.0 (V in dm³). Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, so gas volumes react in the same ratio as moles.

The ideal gas equation

pV = nRT, with SI units:

• p in Pa (1 kPa = 1000 Pa)
• V in m³ (1 dm³ = 10^-3 m³; 1 cm³ = 10^-6 m³)
• T in K (add 273 to °C)
• R = 8.31 J/mol/K

Use n = m / M to find the molar mass of a volatile liquid or gas.

Core Practical 1: molar volume of a gas

React a known small mass of calcium carbonate with excess acid (for example ethanoic acid) and collect the gas in a gas syringe or an upturned measuring cylinder. Calculate moles of gas from the equation, then molar volume = volume / moles. Errors: gas escaping before the bung is replaced, some gas dissolving in water, impure calcium carbonate, and reading the scale.

Worked example

What volume of CO2, at r.t.p., forms when 5.00 g of CaCO3 reacts with excess acid? (Ca = 40.1, C = 12.0, O = 16.0)

1. n(CaCO3) = 5.00 / 100.1 = 0.0500 mol
2. 1 : 1 ratio, so 0.0500 mol CO2
3. V = 0.0500 x 24.0

Answer: 1.20 dm³

Check yourself

1. What volume does 0.250 mol of oxygen occupy at r.t.p.?
   Show answer

   6.00 dm³. V = 0.250 x 24.0 = 6.00 dm³.

2. 0.320 g of a gas occupies 240 cm³ at r.t.p. Find its molar mass.
   Show answer

   32.0 g/mol. n = 240 / 24 000 = 0.0100 mol; M = 0.320 / 0.0100 = 32.0 g/mol.

3. Find the volume, in dm³, of 0.0500 mol of gas at 100 kPa and 300 K. (R = 8.31 J/mol/K)
   Show answer

   1.25 dm³. V = nRT/p = 0.0500 x 8.31 x 300 / 100 000 = 1.25 x 10^-3 m³ = 1.25 dm³.

4. 0.500 g of a volatile liquid vaporises to give 300 cm³ of gas at 101 000 Pa and 373 K. Find its molar mass. (R = 8.31 J/mol/K)
   Show answer

   51.1 g/mol. n = pV/RT = 101 000 x 3.00 x 10^-4 / (8.31 x 373) = 9.775 x 10^-3 mol; M = 0.500 / 0.009775 = 51.1 g/mol.

5. In Core Practical 1, 0.250 g of calcium carbonate reacts with excess ethanoic acid and gives 59.5 cm³ of carbon dioxide. CaCO3 + 2CH3COOH -> Ca(CH3COO)2 + H2O + CO2. Calculate the molar volume in dm³/mol. (Ca = 40.1, C = 12.0, O = 16.0)
   Show answer

   23.8 dm³/mol. n(CaCO3) = n(CO2) = 0.250 / 100.1 = 2.498 x 10^-3 mol; molar volume = 0.0595 / 0.002498 = 23.8 dm³/mol.

6. In pV = nRT, a temperature of 25 °C should be entered as
   • 25 K
   • 248 K
   • 298 K
   • 25 °C
   Show answer

   298 K. Add 273: 25 + 273 = 298 K.

7. In Core Practical 1, which error makes the measured gas volume too small?
   • Some carbon dioxide escapes before the bung is replaced
   • The calcium carbonate is weighed on a balance reading to 0.001 g
   • Excess acid is used
   • The gas syringe is read at eye level
   Show answer

   Some carbon dioxide escapes before the bung is replaced. Lost gas lowers the volume collected, so the molar volume comes out too low.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page
• Pearson IAL Chemistry Unit 1 specimen question paper (constants used)
• Physics and Maths Tutor: IAL Unit 1 notes, Formulae, Equations and Amounts of Substance
• Pearson IAL Chemistry teacher practical guide

Last updated 7 November 2026. Spotted a mistake? Tell us.