A-Level · Chemistry · AQA · Mark scheme decoded

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

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 3 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.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.

What the command words demand

Calculate: Perform a numerical calculation using given data and appropriate formulas.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Determine: Find or derive a specific value or quantity based on given information.
Analyze: Examine and interpret data or graphs to draw conclusions about the behavior of the system.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.

Identify the isotopic masses and abundances.0 marks
35 Cl has a mass of 35 u and an abundance of 75%.37 Cl has a mass of 37 u and an abundance of 25%.
Convert the abundances to decimal form.1 mark
0.75 for 35 Cl0.25 for 37 Cl
Calculate the weighted average.1 mark
A r (Cl) = (0.75 × 35) + (0.25 × 37)
Perform the multiplication and addition.1 mark
A r (Cl) = 26.25 + 9.25
Sum the results to get the relative atomic mass.1 mark
A r (Cl) ≈ 35.5

Final answer: 35.5

Work through every step correctly and you earn all 4 marks.

Another worked example

Calculate the relative molecular mass of carbon dioxide (CO 2 ). Use A r (C) = 12.0 and A r (O) = 16.0.

3 marks

Identify the formula of the molecule and the relative atomic masses of its constituent elements.0 marks
CO 2 consists of one carbon atom and two oxygen atoms.A r (C) = 12.0A r (O) = 16.0
Use the formula for relative molecular mass.1 mark
M r (CO 2 ) = A r (C) + 2 × A r (O)
Substitute the values into the formula.1 mark
M r (CO 2 ) = 12.0 + 2 × 16.0
Perform the multiplication and addition.0 marks
M r (CO 2 ) = 12.0 + 32.0
Sum the results to get the relative molecular mass.1 mark
M r (CO 2 ) = 44.0

Final answer: 44.0

Work through every step correctly and you earn all 3 marks.

Common mistakes

Using the wrong units for atomic masses.
Why it happens: Students often forget that relative atomic mass and relative molecular mass are dimensionless quantities and do not have units like grams or kilograms.
Fix: 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.
Why it happens: Students may overlook the importance of using the natural abundances of isotopes, leading to incorrect weighted averages.
Fix: Always check the given isotopic masses and their natural abundances before performing calculations for A r .
Confusing relative atomic mass with relative molecular mass.
Why it happens: Students sometimes mix up the definitions of A r and M r , leading to incorrect answers in problems involving molecules or compounds.
Fix: 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.
Why it happens: Students may not fully understand that ionic compounds do not form discrete molecules and should use relative formula mass instead of relative molecular mass.
Fix: 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.
Why it happens: Students may make arithmetic errors or forget to include all atoms in the molecule when calculating M r .
Fix: 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.
Why it happens: Students may not fully understand why this specific reference is used, leading to vague or incorrect explanations.
Fix: 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

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Relative Atomic Mass	Calculate an element's relative atomic mass from its isotopic masses and their percentage abundances.	4
Calculate Relative Molecular Mass	Combine relative atomic masses to find the relative molecular mass of a compound.	3
Calculate Relative Formula Mass	Add the relative atomic masses of each atom in the formula to find Mr.	2
Calculate Relative Molecular Mass	Sum the relative atomic masses of all atoms in the glucose molecular formula.	4
Total across these question types		13

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.