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AQA A-Level Chemistry: The Avogadro Constant and Mole Calculations — mark scheme explained

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

The short answer

The concept of the mole is fundamental in chemistry, providing a bridge between the microscopic world of atoms and molecules and the macroscopic quantities we can measure in the laboratory.

The question

Calculate the number of molecules in 0.5 moles of water (H 2 O). [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

    n = 0.5 moles

  • S3

    N A = 6.022 × 10 23 mol -1

  • S4

    Step 2: Use the formula for the number of particles.

  • S5

    N = n × N A

  • S6

    Step 3: Substitute the values into the formula.

  • S7

    N = 0.5 × 6.022 × 10 23

  • S8

    Step 4: Perform the multiplication.

  • S9

    N ≈ 3.011 × 10 23

Model answer

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

  1. S1

    Step 1: Identify the given values.

  2. S2

    n = 0.5 moles

  3. S3

    N A = 6.022 × 10 23 mol -1

  4. S4

    Step 2: Use the formula for the number of particles.

  5. S5

    N = n × N A

  6. S6

    Step 3: Substitute the values into the formula.

  7. S7

    N = 0.5 × 6.022 × 10 23

  8. S8

    Step 4: Perform the multiplication.

  9. S9

    N ≈ 3.011 × 10 23

  10. Final answer: 3.011 × 10 23 molecules

Common mistakes

  • Using the wrong units for mass and volume. — Always ensure that mass is in grams and volume is in cubic decimeters when substituting into the formulas.
  • Forgetting to use the Avogadro constant in particle calculations. — Always include the Avogadro constant (N A ) when calculating the number of particles using the formula N = n × N A .
  • Confusing molar mass with molecular weight. — Remember that molar mass is the mass of one mole of a substance in grams per mole (g mol -1 ). Use this value when converting mass to moles using the formula n = m / M.
  • Incorrectly calculating concentration from volume and amount. — Always use the correct formula for concentration: C = n / V. Double-check your calculations to ensure accuracy.
  • Not reporting results with the appropriate number of significant figures. — Always report your final answer with the same number of significant figures as the least precise measurement in your calculations.
  • Failing to convert volume from cm 3 to dm 3 . — Always convert volume from cubic centimeters (cm 3 ) to cubic decimeters (dm 3 ) by dividing by 1000 when using the formula C = n / V.

Where the marks go

  • Full worked solution (all marking points)3 marks

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