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Practice · Chemistry · 2025 syllabus · S1.5

IB Chemistry S1.5 Ideal gases: practice questions

7 exam-style questions on S1.5 Ideal gases, SL and HL: 4 multiple choice, 1 data-based and 2 structured. Each one comes with the mark scheme, a model answer and the reasoning behind every mark. Free with your email.

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S1.5-01 · Paper 1A style · multiple choice · 1 mark

Closest to ideal behaviour

Under which conditions does the gas behave most like an ideal gas? [1]

  1. Water vapour, H2O(g), at 50 kPa and 500 K
  2. Neon, Ne(g), at 50 kPa and 500 K
  3. Neon, Ne(g), at 500 kPa and 150 K
  4. Water vapour, H2O(g), at 500 kPa and 400 K
Show the answer and the reasoning

Answer: B. B is correct. A gas is closest to ideal when intermolecular attractions are weakest and the particles take up a negligible share of the volume: a non-polar gas with weak London forces, at low pressure (particles far apart) and high temperature (fast particles, attractions matter little). A: right conditions, but water molecules attract strongly through hydrogen bonds. C: high pressure and low temperature push neon away from ideal. D has the polar gas and the high pressure.

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The 7 questions in S1.5

  1. S1.5-01 Closest to ideal behaviourPaper 1A style · multiple choice · 1 mark
  2. S1.5-02 Heating a gas at fixed volumePaper 1A style · multiple choice · 1 mark
  3. S1.5-03 Combined gas lawPaper 1A style · multiple choice · 1 mark
  4. S1.5-04 Why real gases deviatePaper 1A style · multiple choice · 1 mark
  5. S1.5-05 Hydrogen in a gas syringePaper 1B style · data-based · 5 marks
  6. S1.5-06 Pressure of a stored gasPaper 2 style · structured question · 6 marks
  7. S1.5-07 Molar mass of a volatile liquidPaper 2 style · structured question · 6 marks

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.