What this practice covers
These questions are drawn from past CIE 9702 Physics papers and filtered to ideal gases. You answer, you find out immediately whether you were right, and you get the reasoning for the correct option and for each distractor. Wrong answers go to a mistakes locker so you can come back to exactly those.
Practice is free. You need an account only so your progress and your mistakes are still there next time.
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What examiners see students get wrong here
These are the errors that cost marks on ideal gases, taken from our own topic notes. Read them before you practise and you will recognise the traps in the questions.
- Using degrees Celsius in pV = nRT or in E_k = (3/2)kT. Every T in this topic is in kelvin.
- Forgetting that the mole is an SI base unit.
- Mixing up R and k. R is per mole and goes with n; k is per molecule and goes with N.
- Taking the mean of the speeds and then squaring, instead of taking the mean of the squares and then rooting.
- Saying that raising the temperature doubles the r.m.s. speed when it doubles the temperature. Speed goes as the square root.
- Saying that at the same temperature a heavier gas has more kinetic energy per molecule. It has the same, and therefore a lower r.m.s. speed.
- Explaining pressure as molecules "pushing" the walls rather than as the rate of change of momentum in collisions.
- Leaving out the word "random" from the assumptions, or forgetting that collisions must be elastic.
Revise it first
If any of the above is unfamiliar, work through the notes before practising: Ideal gases revision notes.