CIE A Level Physics
CIE A Level Physics Ideal Gases Exam Questions
Free CIE A Level Physics exam questions for Ideal Gases, with a mark scheme for every pack. Covers the mole, the Avogadro constant, the ideal gas equation, kinetic theory assumptions, pressure from molecular motion and mean kinetic energy.
Ideal Gases exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Ideal Gases
Ideal gases is Topic 15 of the Cambridge International AS & A Level Physics (9702) syllabus. It links the macroscopic behaviour of a gas (pressure, volume and temperature) to the motion of its molecules. You use the mole and the Avogadro constant, apply pV = nRT and pV = NkT, state the assumptions of the kinetic theory, derive pV = ⅓Nm<c^2> and show that the mean kinetic energy of a molecule is proportional to thermodynamic temperature. It follows from Temperature and leads into Thermodynamics. Examiners often ask for the derivation and for root-mean-square speed calculations.
What you need to know
- Amount of substance One mole contains NA = 6.02 × 10^23 particles. The number of molecules N = nNA.
- Ideal gas equation pV = nRT or pV = NkT, with R = 8.31 J mol^-1 K^-1 and k = R/NA = 1.38 × 10^-23 J K^-1. T must be in kelvin. An ideal gas obeys this at all pressures, volumes and temperatures.
- Kinetic theory assumptions Molecules are in random motion; collisions are elastic; the volume of the molecules is negligible compared with the volume of the gas; there are no intermolecular forces except during collisions; the time of a collision is negligible compared with the time between collisions.
- Pressure from molecules Molecules change momentum when they hit the walls, producing a force. The derivation gives pV = ⅓Nm<c^2>, where <c^2> is the mean square speed.
- Kinetic energy and temperature Comparing pV = NkT with pV = ⅓Nm<c^2> gives mean kinetic energy ½m<c^2> = (3/2)kT, so mean KE is proportional to thermodynamic temperature.
- Root-mean-square speed c_rms = √<c^2> = √(3kT/m).
Common mistakes in Ideal Gases
- Using temperature in degrees Celsius in pV = nRT. Always convert to kelvin.
- Confusing n (moles) with N (number of molecules) and using R with N or k with n.
- Squaring the mean speed instead of finding the mean of the squared speeds.
- Using the molar mass in kg mol^-1 as the mass of one molecule. Divide by NA to get m.
- Stating 'molecules have no volume'. Say the volume of the molecules is negligible compared with the volume of the container.
Try three Ideal Gases exam-style questions
1. A gas at 1.0 × 10^5 Pa and 290 K occupies 2.4 × 10^-2 m^3. Calculate the amount of gas in moles. (2 marks)
Show the mark scheme answer
n = pV/RT = 1.0 × 10^5 × 2.4 × 10^-2/(8.31 × 290); n = 1.0 mol (0.996 mol).
2. Nitrogen has a molar mass of 0.028 kg mol^-1. Calculate the root-mean-square speed of nitrogen molecules at 300 K. (3 marks)
Show the mark scheme answer
m = 0.028/6.02 × 10^23 = 4.65 × 10^-26 kg; <c^2> = 3kT/m = 3 × 1.38 × 10^-23 × 300/4.65 × 10^-26 = 2.67 × 10^5 m^2 s^-2; c_rms = 517 m s^-1.
3. State three assumptions of the kinetic theory of gases. (3 marks)
Show the mark scheme answer
Molecules move randomly; collisions between molecules and with the walls are elastic; volume of molecules is negligible compared with volume of gas; no forces between molecules except during collisions; collision time negligible compared with time between collisions (any three).
Ideal Gases FAQs
What does Cambridge AS & A Level Physics Ideal gases cover?
It covers the mole and Avogadro constant, the ideal gas equation, the assumptions of kinetic theory, the derivation of pV = ⅓Nm<c^2> and the link between mean kinetic energy and temperature.
Which paper is Ideal gases examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
What equations do I need for Ideal gases?
Learn pV = nRT, pV = NkT, pV = ⅓Nm<c^2>, k = R/NA and mean kinetic energy = (3/2)kT, and practise the kinetic theory derivation step by step.
