A Level Physics (A2) chapter 4: Ideal gases MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

72 exam-style questions on Ideal gases, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

72 questions · 3 topics · 10 per test

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What chapter 4 covers

Ten Ideal gases questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. Copper has density 8900 kg m−3 and molar mass 63.5 g mol−1. How many atoms are in 1.0 cm3 of copper?

  1. A
    8.4 × 1019
  2. B
    8.4 × 1022
  3. C
    8.4 × 1025
  4. D
    5.4 × 1029
Show answer

Answer: B. Mass = 8900 × 1.0 × 10−6 = 8.9 × 10−3 kg; n = 8.9 × 10−3/0.0635 = 0.140 mol; N = 0.140 × 6.02 × 1023 = 8.4 × 1022. The other powers of ten come from converting cm3 or grams wrongly.

2. Gas X has molecules of mass 2m with r.m.s. speed v/2. Gas Y has molecules of mass m with r.m.s. speed v. Both gases have the same number of molecules per unit volume. What is (pressure of X) ÷ (pressure of Y)?

  1. A
    1/4
  2. B
    1/2
  3. C
    1
  4. D
    2
Show answer

Answer: B. p = ⅓(N/V)m⟨c2⟩, so p ∝ m⟨c2⟩: the ratio is 2 × (1/2)2 = 1/2. Forgetting to square the speed ratio gives 1.

3. For a fixed amount n of an ideal gas, a graph of pV (y-axis) against thermodynamic temperature T (x-axis) is drawn. Which describes the graph?

  1. A
    a straight line through the origin with gradient R/n
  2. B
    a straight line with a positive intercept on the pV axis
  3. C
    a curve whose gradient decreases as T increases
  4. D
    a straight line through the origin with gradient nR
Show answer

Answer: D. pV = nRT, so pV is directly proportional to T: a straight line through the origin (pV = 0 at 0 K) with gradient nR.

4. For a fixed amount of an ideal gas kept at 400 K, a graph of pressure p (y-axis) against 1/V (x-axis) is a straight line through the origin with gradient 50 Pa m3. What is the amount of gas?

  1. A
    0.015 mol
  2. B
    0.15 mol
  3. C
    6.0 mol
  4. D
    66 mol
Show answer

Answer: A. p = nRT × (1/V), so the gradient is nRT and n = 50/(8.31 × 400) = 0.015 mol. Inverting the ratio gives 66 mol; leaving out T gives 6.0 mol.

5. A cylinder holds 0.20 mol of nitrogen gas, N2. How many nitrogen atoms does it contain?

  1. A
    6.0 × 1022
  2. B
    1.2 × 1023
  3. C
    2.4 × 1023
  4. D
    4.8 × 1023
Show answer

Answer: C. Molecules: 0.20 × 6.02 × 1023 = 1.2 × 1023; each N2 has two atoms, so there are 2.4 × 1023 atoms. 1.2 × 1023 counts molecules, not atoms.

6. What is meant by an ideal gas?

  1. A
    a gas that obeys pV ∝ θ, where θ is the Celsius temperature
  2. B
    a gas whose pressure is proportional to its volume at constant temperature
  3. C
    a gas that obeys pV ∝ T, where T is the thermodynamic temperature
  4. D
    a gas whose molecules have zero mass
Show answer

Answer: C. An ideal gas obeys pV = nRT at all pressures, volumes and temperatures, with T in kelvin. At constant temperature its pressure is inversely, not directly, proportional to its volume.

7. The product kT, where k is the Boltzmann constant and T is the thermodynamic temperature, has the same unit as which quantity?

  1. A
    Pressure
  2. B
    Power
  3. C
    Momentum
  4. D
    Energy
Show answer

Answer: D. k has the unit J K−1, so kT is in joules: it is an energy on the scale of a single molecule (the mean translational kinetic energy is 3/2 kT).

8. A container of volume 1.0 × 10−3 m3 holds 2.0 × 1022 molecules of an ideal gas at 300 K. What is the pressure of the gas?

  1. A
    83 Pa
  2. B
    8.3 × 104 Pa
  3. C
    8.3 × 107 Pa
  4. D
    5.0 × 1028 Pa
Show answer

Answer: B. p = NkT/V = 2.0 × 1022 × 1.38 × 10−23 × 300/1.0 × 10−3 = 8.3 × 104 Pa. Using R in place of k with the number of molecules gives the absurd 5.0 × 1028 Pa.

9. An oxygen cylinder of volume 0.050 m3 contains gas at a pressure of 2.0 × 106 Pa and a temperature of 17 °C. The molar mass of oxygen is 0.032 kg mol−1. Treating the oxygen as an ideal gas, what is the mass of oxygen in the cylinder?

  1. A
    1.3 kg
  2. B
    23 kg
  3. C
    41 kg
  4. D
    1.3 × 103 kg
Show answer

Answer: A. n = pV/RT = 2.0 × 106 × 0.050/(8.31 × 290) = 41.5 mol, and m = nM = 41.5 × 0.032 = 1.3 kg. 41 is the amount in moles, not a mass; 23 kg comes from using 17 instead of 290 K.

10. One mole of a substance is the amount that contains

  1. A
    a number of particles equal to the Avogadro constant
  2. B
    exactly one gram of the substance
  3. C
    a mass in kilograms equal to its relative molecular mass
  4. D
    a volume of 1 m3 of the substance at room temperature
Show answer

Answer: A. By definition one mole contains NA = 6.02 × 1023 particles. Its mass, the molar mass, is different for different substances; for example it is 0.028 kg for nitrogen gas.

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Ways to practise this chapter

Ideal gases: questions students ask

How many Ideal gases MCQs are there?

72, each with the answer and a short explanation, across 3 topics: The mole, Equation of state, Kinetic theory of gases. Every chapter test draws ten of them, so you can take it many times.

Is the A Level Physics (A2) chapter 4 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

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