A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: Empirical and Molecular Formulae — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 3 July 2026

The short answer

In chemistry, understanding the composition of compounds is fundamental. Two key concepts in this area are empirical formula and molecular formula. These formulas provide different levels of detail about the elements present in a compound and their ratios.

The question

A compound is found to contain 27.3% carbon and 72.7% oxygen by mass. Calculate its empirical formula.

[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.
Determine: Find or derive a specific value or quantity based on given information.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Interpret: Analyze and draw conclusions from provided data or experimental results.

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.

Convert percentages to grams (assume a 100 g sample).1 mark
Carbon = 27.3 g, Oxygen = 72.7 g
Convert grams to moles using the molar masses of carbon and oxygen.1 mark
Moles of Carbon = 27.3 / 12 ≈ 2.28 molesMoles of Oxygen = 72.7 / 16 ≈ 4.54 moles
Find the simplest ratio by dividing each mole value by the smallest number of moles.1 mark
Ratio of Carbon to Oxygen = 2.28 / 2.28 : 4.54 / 2.28 ≈ 1 : 2
The values are already whole numbers, so no further scaling is needed.0 marks
Ratio = 1 : 2
Write the empirical formula.1 mark
Empirical Formula = CO 2

Final answer: CO 2

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

Another worked example

A compound has an empirical formula of CH 2 O and a relative molecular mass of 180 g/mol. Calculate its molecular formula.

3 marks

Determine the molar mass of the empirical formula.1 mark
Molar Mass of CH 2 O = 12 + (2 × 1) + 16 = 30 g/mol
Find the ratio of the molecular mass to the empirical formula mass.1 mark
Ratio = 180 / 30 = 6
Multiply each subscript in the empirical formula by this ratio.1 mark
Molecular Formula = (CH 2 O) 6 = C 6 H 12 O 6

Final answer: C 6 H 12 O 6

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

Common mistakes

Forgetting to convert percentages to grams when calculating empirical formula.
Why it happens: Students often overlook this step, leading to incorrect mole calculations.
Fix: Always assume a 100 g sample and use the given percentages as masses in grams.
Making arithmetic errors when converting grams to moles.
Why it happens: Simple calculation mistakes can lead to incorrect mole values, affecting the final empirical formula.
Fix: Double-check your calculations and use a calculator if necessary.
Not simplifying ratios correctly when finding the empirical formula.
Why it happens: Students may not divide by the smallest number of moles or fail to multiply by a common factor to get whole numbers.
Fix: Always ensure that the ratio is in its simplest form and use multiplication if needed to achieve whole numbers.
Using the wrong molar mass for the empirical formula when calculating the molecular formula.
Why it happens: Students may misinterpret or miscalculate the molar mass of the empirical formula, leading to incorrect ratios and final answers.
Fix: Double-check your calculation of the empirical formula's molar mass and ensure it is accurate.
Forgetting to multiply each subscript in the empirical formula by the ratio when calculating the molecular formula.
Why it happens: Students may skip this step or make arithmetic errors, resulting in an incorrect molecular formula.
Fix: Always multiply each subscript in the empirical formula by the calculated ratio and double-check your final answer.
Misinterpreting experimental data when calculating empirical or molecular formulas.
Why it happens: Students may struggle to extract relevant information from complex data, leading to incorrect calculations.
Fix: Practice analyzing different types of experimental data and focus on extracting the necessary values for your calculations.

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
Empirical Formula Calculation	Use percentage composition to determine the empirical formula of the compound.	4
Determine Molecular Formula	Use empirical formula mass and molecular mass to find the true molecular formula.	3
Empirical Formula Calculation	Determine the simplest whole-number ratio of atoms from percentage composition by mass.	3
Molecular Formula Determination	Use the empirical formula mass and relative molecular mass to find the molecular formula.	3
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.