The questions are on the ideal gas law test. Take it under real conditions before reading the mark scheme.
Quick answer: The ideal gas law, PV = nRT, links pressure, volume, amount and absolute temperature of a gas. This mark scheme gives the model answers and the marks for the 8 questions of the AP Chemistry test on ideal gas law, from gas law basics to gas over water, with the equivalent answers to accept.
This is the ideal gas law test answers for AP Chemistry, with the marks for each answer. First sit the test without notes, then mark it here and revise with the lesson.
Marking points follow each answer. Give method marks for correct substitution and answer marks for correct values with units, allowing small rounding differences. In explanations, accept equivalent wording that names the particle behaviour.
Level: AP Chemistry · Curriculum: AP Chem Unit 3
Ideal gas law test answers: answers and marks
Question 1 Gas law basics (2 marks)
P₁V₁ = P₂V₂, so V₂ = 150 × 8.0 / 400 = 3.0 L.
Marking: 1 correct equation, 1 answer with unit.
Question 2 Ideal gas law (3 marks)
T = 300.15 K. n = PV/RT = 200 × 2.50 / (8.314 × 300.15) = 0.200 mol.
Marking: 1 conversion to kelvin, 1 substitution, 1 answer.
Question 3 Speed distribution (4 marks)
a. Q.
b. The peak of Q is at a higher speed because the average kinetic energy is higher; the curve is lower and wider, since the same number of molecules is spread over more speeds.
c. The area doubles.
Marking: 1 + 2 + 1.
Question 4 Total pressure of a mixture (2 marks)
P = nRT/V = 0.500 × 8.314 × 300 / 5.00 = 249 kPa.
Marking: 1 total amount, 1 pressure with unit.
Question 5 Partial pressure of helium (2 marks)
x(He) = 0.200/0.500 = 0.400; P(He) = 0.400 × 249.4 = 99.8 kPa.
Marking: 1 mole fraction, 1 answer.
Question 6 Why gases deviate (3 marks)
At high pressure particles are close, so their own volume is no longer negligible and the volume is larger than the ideal value. At low temperature particles move slowly and attractions pull them together, so the pressure is lower than the ideal value.
Marking: 1 particle volume, 1 attractions, 1 link to pressure or volume.
Question 7 Molar mass (2 marks)
n = 100 × 0.500 / (8.314 × 300) = 0.02005 mol; M = 0.800 / 0.02005 = 39.9 g/mol.
Marking: 1 amount, 1 molar mass.
Question 8 Gas over water (2 marks)
The collected gas is mixed with water vapour. The total pressure is the sum of the partial pressures, so P(gas) = P(total) − P(water vapour).
Marking: 1 mixture, 1 Dalton’s law.
