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AQA A-Level Chemistry: Relative Atomic Mass and Relative Molecular Mass — mark scheme explained

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

The short answer

In chemistry, understanding the relative masses of atoms and molecules is crucial for a variety of calculations and concepts. This section focuses on defining and calculating relative atomic mass (A r ) and relative molecular mass (M r ).

The question

Calculate the relative atomic mass of chlorine, given that it has two isotopes: 35 Cl (75% abundance) and 37 Cl (25% abundance). [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 isotopic masses and abundances.

  • S2

    35 Cl has a mass of 35 u and an abundance of 75%.

  • S3

    37 Cl has a mass of 37 u and an abundance of 25%.

  • S4

    Step 2: Convert the abundances to decimal form.

  • S5

    0.75 for 35 Cl

  • S6

    0.25 for 37 Cl

  • S7

    Step 3: Calculate the weighted average.

  • S8

    A r (Cl) = (0.75 × 35) + (0.25 × 37)

  • S9

    Step 4: Perform the multiplication and addition.

  • S10

    A r (Cl) = 26.25 + 9.25

  • S11

    Step 5: Sum the results to get the relative atomic mass.

  • S12

    A r (Cl) ≈ 35.5

Model answer

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

  1. S1

    Step 1: Identify the isotopic masses and abundances.

  2. S2

    35 Cl has a mass of 35 u and an abundance of 75%.

  3. S3

    37 Cl has a mass of 37 u and an abundance of 25%.

  4. S4

    Step 2: Convert the abundances to decimal form.

  5. S5

    0.75 for 35 Cl

  6. S6

    0.25 for 37 Cl

  7. S7

    Step 3: Calculate the weighted average.

  8. S8

    A r (Cl) = (0.75 × 35) + (0.25 × 37)

  9. S9

    Step 4: Perform the multiplication and addition.

  10. S10

    A r (Cl) = 26.25 + 9.25

  11. S11

    Step 5: Sum the results to get the relative atomic mass.

  12. S12

    A r (Cl) ≈ 35.5

  13. Final answer: 35.5

Common mistakes

  • Using the wrong units for atomic masses. — Always remember that A r and M r are dimensionless and should not be given units in calculations.
  • Forgetting to use the correct isotopic abundances when calculating relative atomic mass. — Always check the given isotopic masses and their natural abundances before performing calculations for A r .
  • Confusing relative atomic mass with relative molecular mass. — Remember that A r is for individual atoms, while M r is for molecules. For ionic compounds, use the term 'relative formula mass'.
  • Misinterpreting the empirical formula of an ionic compound. — Practice identifying whether a substance is a molecule or an ionic compound. Use M r for both, but remember the difference in context.
  • Incorrectly summing the relative atomic masses to find the relative molecular mass. — Double-check your calculations and ensure that you have included all atoms in the formula. Use a step-by-step approach to avoid mistakes.
  • Failing to explain the significance of using one-twelfth the mass of a carbon-12 atom as a reference. — Practice explaining that the use of one-twelfth the mass of a carbon-12 atom provides a standard scale for comparing the masses of different elements and molecules.

Where the marks go

  • Full worked solution (all marking points)4 marks

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