Explain why the molar volume of a real gas at high pressure is greater than the value predicted by the ideal gas equation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The ideal gas equation assumes that the gas particles have no volume and that there are no intermolecular forces. At high pressure the particles are forced very close together, so the finite volume of the particles becomes a significant fraction of the total volume. Consequently the actual volume available for the particles to move in is larger than the volume predicted by PV=nRT. In addition, the short‑range repulsive forces between the particles become important at close distances, pushing the particles apart and further increasing the volume beyond the ideal prediction.
Examiner tips
- Use the term ‘finite molecular volume’ and ‘short‑range repulsive forces’.
- Explain that the ideal gas model neglects particle volume and forces.
- Show the logical link: high pressure → particles close → volume occupied by particles significant → actual volume > ideal.
- Mention repulsive forces as an additional cause of volume increase.
Common mistakes
- Confusing ‘greater’ with ‘smaller’ volume; the real gas volume is larger, not smaller.
- Forgetting to mention the finite volume of the particles or the repulsive forces.
- Using vague phrases like ‘molecules are close together’ without linking to volume or forces.
Mark scheme (4 marks)
- The ideal gas model assumes that gas molecules/particles occupy negligible/zero volume compared to the total volume of the container.
- At high pressure, gas molecules are forced close together so the volume occupied by the molecules themselves becomes a significant fraction of the total volume.
- The actual/real volume available for movement is therefore greater than predicted by PV = nRT, because the molecules themselves take up space.
- At high pressure the repulsive (short-range) intermolecular forces also become significant, pushing molecules apart and further increasing the volume beyond the ideal prediction.
Key terms in this question
molar volume · ideal gas equation · real gas · high pressure
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Ideal gases questions
- Explain why the pressure exerted by a real gas is lower than that predicted by t…
- Explain why, at very high temperatures and low pressures, the behaviour of a rea…
- Explain why the ideal gas equation becomes less accurate at low temperatures whe…
- Explain why a real gas such as ammonia deviates more significantly from ideal ga…