3.8 Gas pressure, volume and doing work on a gas
9 exam questions worth 22 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A gas has a pressure of 100 kPa and a volume of 60 cm³. It is compressed to 20 cm³ at constant temperature. Calculate the new pressure.
Mark scheme
p₁V₁ = p₂V₂ (1); 100 × 60 = p₂ × 20 (1); 300 kPa (1)
Model answer
p₁V₁ = p₂V₂, so p₂ = 100 × 60 / 20 = 300 kPa.
A balloon holds 2.0 m³ of gas at a pressure of 150 kPa. It rises and expands to 6.0 m³ at the same temperature. Calculate the new pressure.
Mark scheme
p₁V₁ = p₂V₂ (1); 150 × 2.0 = p₂ × 6.0 (1); 50 kPa (1)
Model answer
p₂ = p₁V₁ / V₂ = 150 × 2.0 / 6.0 = 50 kPa.
Use the particle model to explain why the pressure of a gas increases when its volume is decreased at constant temperature.
Mark scheme
the same number of particles are in a smaller volume / are closer together (1); the particles hit the walls more often (1); so there is a greater force on each unit area / pressure increases (1)
Model answer
The same number of particles are in a smaller space, so they hit the walls more often. The force on each unit area is greater, so the pressure increases.
When a bicycle pump is used quickly, the barrel gets warm. Explain why. (Higher tier)
Mark scheme
work is done on the gas when it is compressed (1); this increases the internal energy / kinetic energy of the particles, so the temperature of the gas rises (1)
Model answer
Work is done on the air when it is compressed. This increases the internal energy of the air, and so the temperature of the gas rises.
State the direction of the force that a gas exerts on the walls of its container.
Mark scheme
at right angles to the wall / surface
Model answer
The force acts at right angles to the surface of the wall.
The volume of a fixed mass of gas is doubled at constant temperature. What happens to its pressure?
Choose one answer.
Correct Not quite: the answer is It halves
Mark scheme
B: it halves (pV is constant)
Model answer
It halves (B), because pressure × volume is constant.
A gas has a volume of 600 cm³ at a pressure of 100 kPa. It is compressed to a volume of 150 cm³ at constant temperature. Calculate the new pressure.
Mark scheme
p₁V₁ = p₂V₂ (1); p₂ = 100 × 600 ÷ 150 (1); 400 kPa (1)
Model answer
p₂ = p₁V₁ / V₂ = 100 × 600 / 150 = 400 kPa.
A bubble of air has a volume of 1.5 cm³ at the bottom of a pond, where the pressure is 250 kPa. It rises to the surface, where the pressure is 100 kPa. Calculate the volume of the bubble at the surface, assuming constant temperature.
Mark scheme
V₂ = p₁V₁ ÷ p₂ (1); 250 × 1.5 ÷ 100 (1); 3.75 cm³ (1)
Model answer
V₂ = 250 × 1.5 / 100 = 3.75 cm³.
(Higher) An aerosol can feels cold when the spray has been used for a while. Explain why.
Mark scheme
the gas expands as it leaves the can (1); it does work on the surroundings / pushes the air out of the way (1); so its internal energy decreases and its temperature falls (1)
Model answer
The gas expands as it leaves the can, doing work on the surroundings. This reduces its internal energy, so it cools down, and the can feels cold.
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