1. The ideal gas equation
| Symbol | Quantity | SI unit |
|---|---|---|
| p | Pressure | Pa |
| V | Volume | m3 |
| n | Amount | mol |
| R | Gas constant (given, e.g. 8.31) | J mol−1 K−1 |
| T | Temperature | K |
2. Unit conversions
- kPa → Pa: × 1000 (100 kPa = 1.00 × 105 Pa)
- cm3 → m3: × 10−6; dm3 → m3: × 10−3
- °C → K: + 273.15 (AQA questions often use + 273). Write "K", not "°K".
Getting V wrong by a factor of 1000 is one of the most common errors. Convert first, then substitute.
3. Worked calculations
4. Real gases and mixtures
Real gases behave most like ideal gases at low pressure and high temperature, where the particles are far apart and forces between them are small. In a gas mixture, n in pV = nRT is the total moles of all gases.
The molar gas volume of about 24 dm3 mol−1 applies only at around room temperature and pressure. Use it only when the question gives it; otherwise use pV = nRT.
5. Reacting gas volumes
At the same temperature and pressure, equal volumes of gases contain equal numbers of moles. So gas volumes react in the same ratio as the coefficients in the equation.
If water is a product and the gases are cooled back to room temperature, exam questions usually treat the water as condensed and ignore water vapour, so it is not counted in the gas volume, unless the question says it is steam.
6. Gas-syringe practicals
A gas syringe collects gas without water. It needs an airtight connection, a smoothly moving plunger and clear graduations. Record the temperature and pressure.
| Problem | Effect |
|---|---|
| Gas escapes before the bung is fitted | Volume too low, so n too low; if mass is known, calculated M too high |
| Volatile liquid not fully vaporised | Volume too low, so calculated M too high |
| Gas dissolves in the reaction solution | Volume too low |
| Collecting over water | Only suitable if the gas is not very soluble in water |
Diagram placeholder
Collecting gas in a gas syringe
Labels to include:
- Conical flask with reactants
- Bung and delivery tube (airtight)
- Gas syringe with graduations
- Plunger moving out as gas collects
- Thermometer / record of room T and p
Gas from the flask passes through the delivery tube and pushes the plunger out; the volume is read from the syringe scale.
7. Quick checks
Finesse practice: indicative answers to check your reasoning, not official mark allocations. R = 8.31 J mol−1 K−1.
Q1. Find n for 250 cm³ of gas at 98.0 kPa and 50 °C (use 323 K).Show answer
p = 9.80 × 104 Pa; V = 2.50 × 10−4 m3
n = (9.80 × 104 × 2.50 × 10−4) ÷ (8.31 × 323) = 24.5 ÷ 2684.1 = 9.13 × 10−3 mol
Q2. What volume (in cm³) does 0.0100 mol of gas occupy at 100 kPa and 300 K?Show answer
V = (0.0100 × 8.31 × 300) ÷ (1.00 × 105) = 2.49 × 10−4 m3
× 106 = 249 cm3
Q3. 20 cm³ CH₄ burns in 50 cm³ O₂: CH₄ + 2O₂ → CO₂ + 2H₂O. After cooling, with volumes measured at the same T and p as at the start, what is the total gas volume?Show answer
O2 used = 40 cm3, so 10 cm3 O2 is left. CO2 formed = 20 cm3. The water condenses and is ignored (water vapour neglected).
Total = 30 cm3
Q4. 0.240 g of a gas occupies 140 cm³ at 101 kPa and 298 K. Find its Mr.Show answer
n = (1.01 × 105 × 1.40 × 10−4) ÷ (8.31 × 298) = 14.14 ÷ 2476.4 = 5.710 × 10−3 mol
Mr = 0.240 ÷ (5.710 × 10−3) = 42.0
Q5. In finding Mr from a known mass of gas, some gas escaped before the syringe was connected. Effect?Show answer
Measured volume too small, so n is too small. M = mass ÷ n, so the calculated Mr is too high.
8. Sources
Sources and examiner guidance (reviewed 1 October 2026)
- Chemrevise — AQA 1.2 Calculations (amount of substance) revision guide (N. Goalby) — primary content reference (ideal gas equation, gas volumes)
- AQA 7405 specification — 3.1.2 Amount of substance (incl. Required Practical 1) — 3.1.2.3 The ideal gas equation
- AQA 7404/1 mark scheme, June 2023 — Q04: SI units, total gas moles, ratio, then Mr and Ar
- AQA 7404/1 examiner report, June 2023 — Q04: unit conversion, rearrangement and ratio errors
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.