A-Level · Physics · AQA · Mark scheme decoded

AQA A-Level Physics: Gas Laws and Ideal Gas Equation — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Physics specificationlast verified 2 July 2026

The short answer

The study of gas laws is fundamental to understanding the behavior of gases under various conditions. These laws describe how pressure (p), volume (V), temperature (T), and mass of a gas are related.

The question

A sealed container with a volume of 2 m 3 contains an ideal gas at a temperature of 300 K. The pressure inside the container is 100,000 Pa. Calculate the number of moles of gas in the container. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

S1
Identify the given values: p = 100,000 Pa, V = 2 m 3 , T = 300 K, R = 8.314 J/(mol·K).
S2
Use the ideal gas equation for moles of gas: pV = nRT.
S3
Rearrange to solve for n: n = (pV) / (RT).
S4
Substitute the values: n = (100,000 × 2) / (8.314 × 300).
S5
Calculate: n ≈ 80.6 moles.

Model answer

Worked through, with each step tagged to the mark it earns.

S1
Identify the given values: p = 100,000 Pa, V = 2 m 3 , T = 300 K, R = 8.314 J/(mol·K).
S2
Use the ideal gas equation for moles of gas: pV = nRT.
S3
Rearrange to solve for n: n = (pV) / (RT).
S4
Substitute the values: n = (100,000 × 2) / (8.314 × 300).
S5
Calculate: n ≈ 80.6 moles.
Final answer: 80.6 moles

Common mistakes

Confusing the units of pressure and volume in the ideal gas equation. — Always double-check the units before substituting values into the ideal gas equation. Ensure pressure is in Pascals (Pa) and volume is in cubic meters (m 3 ).
Using absolute temperature in Celsius instead of Kelvin. — Always use absolute temperature (Kelvin) in the ideal gas equation. Convert Celsius to Kelvin by adding 273.15.
Forgetting to convert volume from liters to cubic meters. — Convert volume from liters to cubic meters by dividing by 1000. For example, 1 liter = 0.001 m 3 .
Misapplying Boyle's Law or Charles's Law in practical experiments. — Ensure that the correct conditions are maintained during the experiment. For Boyle's Law, keep the temperature constant; for Charles's Law, keep the pressure constant.
Confusing molar mass with molecular mass. — Understand the difference between molar mass and molecular mass. Molar mass is the mass of one mole of a substance, while molecular mass is the mass of one molecule.
Incorrectly calculating work done by a gas. — Use the correct formula W = pΔV and pay attention to the sign of ΔV. If the volume decreases (compression), ΔV is negative; if the volume increases (expansion), ΔV is positive.

Where the marks go

Full worked solution (all marking points)5 marks