How is a partial pressure found?

Get the total pressure from the total number of moles, then share it in proportion to each gas mole fraction.

Why does a released cylinder expand so much?

At constant temperature the product of pressure and volume is fixed, so a two hundredfold drop in pressure gives a two hundredfold rise in volume.

S5 Physics Gas Laws, Paper 4: moles, partial pressure and cylinder release

2026-11-02

Tutor Prep Guide · Primary English

Finds amount from pV equals nRT, works out gas density and partial pressures, then handles cylinder release and a heating plus compression in ten graded items.

Focus　Convert to kelvin, cubic metres, pascals and moles before substituting, and split a mixture by mole fraction.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Gas Laws
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationFind the volume of 0.5 mol of ideal gas at 300 K and 100 kPa, with R as 8.31.
FoundationAt constant pressure 3 L of gas is heated from 300 K to 400 K. Find the volume.
FoundationAt constant volume the pressure is 90 kPa at 300 K. Find it at 350 K.
FoundationHow many moles are there in 0.02 cubic metres at 2.0 times ten to the five pascals and 400 K?
IntermediateA gas at 100 kPa in 0.3 cubic metres and 300 K is compressed to 0.1 cubic metres at 350 K. Find the pressure.
IntermediateTwo moles of carbon dioxide, molar mass 44, fill a 50 litre vessel. Find the density in grams per litre.
Intermediate2 mol of nitrogen and 3 mol of oxygen share 0.05 cubic metres at 300 K. Find the total pressure and the oxygen partial pressure.
ChallengeA 10 L cylinder holds gas at 2.0 times ten to the seven pascals. Find the volume it fills at 1.0 times ten to the five pascals at constant temperature.
ChallengeA gas at 1.2 times ten to the five pascals in 2 L at 300 K is heated to 450 K and squeezed into 1.5 L. Find the pressure.
Error redoA student uses 47 instead of 320 in pV equals nRT and gets 0.0078 cubic metres for 3 mol at 47 C and 1.5 times ten to the five pascals. Find the correct volume.

03　Answers and one-line notes

#	Answer	One-line note
1	0.0125 cubic metres, that is 12.5 L	Half a mole at 300 K and 100 kPa takes about 0.0125 cubic metres, that is 12.5 L.
2	4 L	At constant pressure volume over kelvin is fixed, so 3 times 400 over 300 gives 4 L.
3	105 kPa	At constant volume pressure over kelvin is fixed, so 90 times 350 over 300 gives 105 kPa.
4	1.20 mol	Amount is pressure times volume over R times T, giving about 1.20 mol.
5	350 kPa	The combined gas law gives 100 times 0.3 times 350 over 0.1 times 300, that is 350 kPa.
6	1.76 grams per litre	Two moles weigh 88 g, so the density is 88 over 50, that is 1.76 g per litre.
7	249.3 kPa in total, with oxygen at 149.6 kPa	Five moles in total give about 249300 pascals, and oxygen takes three fifths of that, about 149580 pascals.
8	2000 L	At constant temperature the product of pressure and volume is fixed, so the gas expands to 2000 litres.
9	2.4 times ten to the five pascals	Pressure rises with kelvin and with the volume ratio, giving 2.4 times ten to the five pascals.
10	0.0532 cubic metres, that is 53.2 L	Kelvin must be used, so 320 K gives about 0.0532 cubic metres instead of 0.0078.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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