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AQA A-Level Chemistry: The Ideal Gas Equation — mark scheme explained

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

The short answer

The ideal gas equation, pV = nRT, is a fundamental concept in physical chemistry that describes the behavior of gases under various conditions. This equation relates four key variables: pressure (p), volume (V), amount of substance (n), and temperature (T).

The question

A sample of an ideal gas has a volume of 0.5 m 3 , contains 2 moles, and is at a temperature of 300 K. Calculate the pressure of the gas. [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

    Step 1: Identify the given values.

  • S2

    V = 0.5 m 3

  • S3

    n = 2 mol

  • S4

    T = 300 K

  • S5

    R = 8.314 J/(mol·K)

  • S6

    Step 2: Use the ideal gas equation to find pressure (p).

  • S7

    pV = nRT

  • S8

    p = nRT / V

  • S9

    Step 3: Substitute the values into the formula.

  • S10

    p = (2 mol × 8.314 J/(mol·K) × 300 K) / 0.5 m 3

  • S11

    Step 4: Perform the multiplication and division.

  • S12

    p = (4988.4 J) / 0.5 m 3

  • S13

    p = 9976.8 Pa

Model answer

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

  1. S1

    Step 1: Identify the given values.

  2. S2

    V = 0.5 m 3

  3. S3

    n = 2 mol

  4. S4

    T = 300 K

  5. S5

    R = 8.314 J/(mol·K)

  6. S6

    Step 2: Use the ideal gas equation to find pressure (p).

  7. S7

    pV = nRT

  8. S8

    p = nRT / V

  9. S9

    Step 3: Substitute the values into the formula.

  10. S10

    p = (2 mol × 8.314 J/(mol·K) × 300 K) / 0.5 m 3

  11. S11

    Step 4: Perform the multiplication and division.

  12. S12

    p = (4988.4 J) / 0.5 m 3

  13. S13

    p = 9976.8 Pa

  14. Final answer: 9976.8 Pa

Common mistakes

  • Forgetting to convert units before using them in the ideal gas equation. — Always check and convert units to pascals (Pa) for pressure, cubic meters (m 3 ) for volume, moles (mol) for amount of substance, and kelvins (K) for temperature before substituting them into the equation.
  • Using the wrong value for the gas constant R. — Always use the correct value of R (8.314 J/(mol·K) or 0.0821 L·atm/(mol·K)) and ensure it matches the units of the other variables.
  • Incorrectly rearranging the ideal gas equation. — Practice rearranging the equation step-by-step and double-check your work to ensure accuracy.
  • Misinterpreting the relationship between variables in the ideal gas equation. — Practice problems that involve changing one variable and observing the effect on others. Understand the direct and inverse relationships between pressure, volume, amount of substance, and temperature.
  • Using the wrong formula for conversions. — Memorize common conversion factors (e.g., 1 L = 0.001 m 3 , 1 atm = 101325 Pa) and double-check your conversions.
  • Forgetting to use absolute temperature (K). — Always convert temperatures to kelvins by adding 273.15 to the Celsius temperature before using them in the ideal gas equation.

Where the marks go

  • Full worked solution (all marking points)4 marks

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