Free B.3 Gas laws questions for IB Physics (SL & HL), part of B The particulate nature of matter. Each question has the answer and the marking points an examiner would look for.
Generate a full B.3 Gas Laws worksheet →All B The particulate nature of matter questionsAn ideal gas is compressed slowly at constant temperature. What happens to the mean speed of the molecules and the rate of collisions with each unit area of the walls?
| mean speed | collision rate per unit area | |
|---|---|---|
| A | unchanged | unchanged |
| B | increases | increases |
| C | increases | unchanged |
| D | unchanged | increases |
Answer: D
An ideal gas at pressure 2.0 × 105 Pa and temperature 290 K occupies a volume of 5 × 10−3 m3.
How many molecules are there in the gas?
Answer: C
A fixed mass of air is trapped in a syringe at a room temperature of 20 °C. The volume V is changed slowly and the pressure p is read from a gauge.
| V / cm3 ± 1 | 20 | 25 | 30 | 40 | 50 |
|---|---|---|---|---|---|
| p / kPa ± 5 | 364 | 293 | 245 | 183 | 146 |
Explain why the volume should be changed slowly.
State what graph the student should plot to obtain a straight line through the origin.
Use the graph to determine the number of moles of air in the syringe.
Determine the percentage uncertainty in V for the smallest volume, and state whether it or the uncertainty in p is more significant.
Air can leak past the plunger at high pressure. State and explain the effect of this on the graph.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | keep temperature constant so that the temperature of the gas stays constant (isothermal) ✓ | 1 |
| (b) | p against 1/V p against 1V ✓ | 1 |
| (c) | 3.0 × 10−3 mol gradient = pV = 7.29 kPa × 103 cm3 ✓ = 7.29 J (unit conversion) ✓ n = pVRT ✓ n = 3.0 × 10−3 mol ✓ | 4 |
| (d) | 5.00% 120 × 100 = 5.00% ✓ compared with 5364 × 100 = 1.37% for p, so V is more significant ✓ | 2 |
| (e) | gradient falls n decreases at high pressure (small V, large 1/V) ✓ so points at high 1/V fall below the line; the graph curves downwards ✓ | 2 |
A weather balloon contains 3.0 m3 of helium at a pressure of 100 kPa and 290 K at ground level. It rises to a height where the pressure is 50 kPa and the temperature is 250 K. Assume helium is an ideal gas and the balloon exerts negligible pressure.
Calculate the number of moles of helium in the balloon.
Calculate the volume of the balloon at this height.
Calculate the change in the average kinetic energy of a helium atom.
Explain, using the kinetic model, why the pressure inside the balloon decreases as it rises.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 124 mol n = pV/RT ✓ = 124 mol ✓ | 2 |
| (b) | 5.17 m3 p1V1/T1 = p2V2/T2 ✓ V2 = 5.17 m3 ✓ | 2 |
| (c) | −8.3 × 10−22 J ΔE = 3/2 kBΔT ✓ = −8.3 × 10−22 J ✓ | 2 |
| (d) | fewer, slower collisions volume increases so molecules hit unit area of the wall less often ✓ temperature falls so the momentum change per collision is smaller ✓ | 2 |
Nitrogen molecules each have a mass of 4.65 × 10−26 kg. The gas is at 300 K.
Calculate the root mean square speed of the molecules.
State two assumptions of the kinetic model of an ideal gas.
State the temperature at which the rms speed would be doubled.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 520 m s−1 ½mv2 = 3/2 kBT ✓ v = 517 m s−1 ✓ | 2 |
| (b) | any two molecules have negligible volume ✓ collisions are elastic / no intermolecular forces except in collisions / random motion ✓ | 2 |
| (c) | 1200 K 1200 K ✓ | 1 |
A sealed container holds 0.050 mol of an ideal gas at 300 K and 1.2 × 105 Pa.
Calculate the volume of the container.
The gas is heated to 450 K at constant volume. Calculate the new pressure.
Explain, using the kinetic model, why the pressure increases.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 1.0 × 10−3 m3 V = nRT/p ✓ 1.04 × 10−3 m3 ✓ | 2 |
| (b) | 1.8 × 105 Pa 1.8 × 105 Pa ✓ | 1 |
| (c) | faster molecules, more frequent harder collisions molecules move faster (greater mean KE) ✓ collide with walls more often and with greater change of momentum ✓ | 2 |
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Meet Riyaz →Original IB-style practice questions written for mentorriyaz.in. These are not IB past paper questions, and this site is not affiliated with or endorsed by the International Baccalaureate Organization. Updated 2026-11-04. Paper generator · mentorriyaz.in