The questions are on the ideal gas law test. Take it under real conditions before reading the mark scheme.
Quick answer: An ideal gas is a collection of tiny, randomly moving particles whose average kinetic energy is proportional to the kelvin temperature. This mark scheme gives the model answers and the marks for the 8 questions of the AP Physics 2 test on ideal gas law, from kinetic model to compressing air, with the equivalent answers to accept.
This is the ideal gas law test answers for AP Physics 2, with the marks for each answer. First sit the test without notes, then mark it here and revise with the lesson.
The model answers show each step. In explanations, marks go to the correct physical idea in particle terms, not to long wording. Penalise only once for a wrong temperature scale and give follow-through credit when later parts use an earlier wrong value correctly.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 9
Ideal gas law test answers: answers and marks
Question 1 Kinetic model (2 marks)
- Particles have negligible volume; there are no forces between particles except in collisions (also acceptable: elastic collisions, random motion).
- Each collision gives the wall an impulse; the large number of collisions per second gives a steady average force per unit area.
Marking: 1 mark each part.
Question 2 Particles in a tank (3 marks)
- n = pV/(RT) = (1.5 × 10⁵ × 4.0 × 10⁻³)/(8.31 × 300) = 0.24 mol.
- N = nNA = 0.241 × 6.02 × 10²³ = 1.4 × 10²³.
Marking: Part a: formula and unit conversion (1), answer (1). Part b: 1 mark.
Question 3 Temperature and speed (2 marks)
- Kelvin temperatures are 300 K and 600 K, so the ratio is 2.0.
- vrms ∝ √T, so the ratio is √2 = 1.41.
Marking: 1 mark each part; deduct 1 for using Celsius.
Question 4 A pressure graph (3 marks)
- Pressure is proportional to kelvin temperature at constant V and n; at 0 K the particles have no kinetic energy and exert no pressure.
- (128 − 80) kPa/(400 − 250) K = 0.32 kPa/K = 320 Pa/K.
- gradient = nR/V, so n = 320 × 5.0 × 10⁻³/8.31 = 0.19 mol.
Marking: 1 mark per part.
Question 5 A cool box lid (3 marks)
- H = kAΔT/L = (0.033 × 0.30 × 26)/0.040 = 6.4 W.
- The rate halves, to about 3.2 W.
Marking: Part a: substitution (1), answer with unit (1). Part b: 1 mark.
Question 6 Metal and wood (3 marks)
- Steel has a much larger thermal conductivity, so heat flows from the hand into the handle at a much higher rate. The skin loses energy faster and you sense this faster cooling as a lower temperature.
- Steel.
Marking: Part a: larger k (1), larger heat flow rate from the hand (1). Part b: 1 mark.
Question 7 Faulty reasoning (2 marks)
- The temperature must be in kelvin, and vrms depends on the square root of the kelvin temperature.
- √(313/293) = 1.034, an increase of about 3.4%.
Marking: 1 mark each part.
Question 8 Compressing air (2 marks)
- The pressure doubles.
- The particles keep the same speed but the container is half as long, so they hit the walls twice as often, doubling the average force per unit area.
Marking: 1 mark each part.
