Cambridge O Level Chemistry · Syllabus 5070 · Stoichiometry
Molar gas volume
What is Molar gas volume?
The molar gas volume is the volume occupied by one mole of any gas under stated conditions; for Cambridge O Level Chemistry it is taken as 24 cubic decimetres per mole at room temperature and pressure, equivalently 24 000 cubic centimetres per mole.
This definition is part of the Stoichiometry chapter in Cambridge O Level Chemistry.
Common mistakes with Molar gas volume
- "Multiply the mass by 24." Fix The molar gas volume multiplies an amount, not a mass. Convert with \(n = m/M\) first, then multiply.
Questions students ask about Molar gas volume
When do you use 24 and when do you use 24 000 in a gas volume calculation?
The molar gas volume at room temperature and pressure (r.t.p.) is \(24\ \mathrm{dm^3}\), which is the same as \(24\,000\ \mathrm{cm^3}\). Read the unit on the volume you were given, then choose: 24 goes with \(\mathrm{dm^3}\) and 24 000 goes with \(\mathrm{cm^3}\). Mixing them is a factor-of-1000 error that looks plausible on a calculator. The same rule applies to concentration: \(n = cV\) needs \(V\) in \(\mathrm{dm^3}\), so divide a \(\mathrm{cm^3}\) volume by 1000 on its own line before you substitute. Always state at r.t.p. in the answer.