Explain why two different ideal gases, when mixed together in a sealed container at constant temperature, exert a greater total pressure than either gas alone occupying the same container.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sealed rigid container holds a mixture of two ideal gases, nitrogen and argon, at room temperature.
Model answer (4 marks)
Each gas exerts its own partial pressure independently of the other gas (Dalton’s law). The total pressure in the container is the algebraic sum of the partial pressures of nitrogen and argon. In an ideal gas the molecules have negligible volume and no intermolecular forces, so adding more molecules to the same volume increases the frequency of collisions with the walls, raising the pressure. Because the gases are ideal, there are no forces between particles of different gases, so each gas behaves independently and its contribution to pressure is unchanged by the presence of the other gas.
Examiner tips
- Use the phrase ‘Dalton’s law of partial pressures’ to show understanding of independence of gases.
- Show that total pressure = sum of partial pressures and explain why adding molecules increases collisions.
- Mention ideal gas assumptions (negligible volume, no forces) to justify independence.
- Keep answer concise and directly linked to the four scheme points.
Common mistakes
- Confusing total pressure with the pressure of one gas only.
- Failing to mention Dalton’s law explicitly.
- Assuming intermolecular forces exist between the gases.
Mark scheme (4 marks)
- Each gas exerts its own partial pressure independently of the other gas (Dalton's law of partial pressures).
- Total pressure equals the sum of the individual partial pressures of all gases present.
- In an ideal gas, molecules have negligible volume and no intermolecular forces, so adding more molecules to the same volume increases the frequency of collisions with the container walls.
- Because the gases are ideal, there are no intermolecular forces between particles of different gases, so each gas behaves independently and its contribution to pressure is unaffected by the presence of the other gas.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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