Relative Formula Mass
Get to grips with Relative Formula Mass for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Relative formula mass (Mr) is the sum of the relative atomic masses of all the atoms shown in a chemical formula.
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Topic overview
The relative formula mass of a compound, written Mr, is the sum of the relative atomic masses of all the atoms shown in its formula.
To calculate it, multiply each element's relative atomic mass by the number of its atoms in the formula, then add the results. For water H₂O the calculation is (2 × 1) + 16 = 18.
Brackets need care. In calcium hydroxide Ca(OH)₂ the subscript 2 applies to everything inside the bracket, so there are two oxygen atoms and two hydrogen atoms, giving 40 + (2 × 16) + (2 × 1) = 74. Because mass is conserved, the total Mr of the reactants always equals the total Mr of the products.
Revision notes
The calculation
Multiply each element's relative atomic mass by the number of its atoms, then add the results.
Relative atomic masses are read from the periodic table — they are the larger of the two numbers shown for each element.
Handling brackets
A subscript outside a bracket multiplies everything inside it.
In Ca(OH)₂ there are one calcium, two oxygen and two hydrogen atoms. In Al₂(SO₄)₃ there are two aluminium, three sulfur and twelve oxygen atoms.
Conservation check
In a balanced equation, the sum of the relative formula masses of the reactants equals that of the products.
This provides a useful check: if the two totals differ, either the equation is not balanced or an Mr has been calculated incorrectly.
Key points
- Mr is the sum of the relative atomic masses in a formula.
- Multiply each atomic mass by the number of atoms.
- Relative atomic mass is the larger number in the periodic table.
- A subscript outside a bracket multiplies everything inside.
- Total Mr of reactants equals total Mr of products.
- Mr has no units.
Worked examples
Example 1
Calculate the relative formula mass of CO₂. Ar: C = 12, O = 16. [2 marks]
Model answer
Carbon contributes 12 and the two oxygen atoms contribute 2 × 16 = 32 (1 mark). Mr = 12 + 32 = 44 (1 mark).
Example 2
Calculate the relative formula mass of Ca(OH)₂. Ar: Ca = 40, O = 16, H = 1. [3 marks]
Model answer
The subscript 2 applies to both atoms inside the bracket, giving two oxygen and two hydrogen atoms (1 mark). Oxygen contributes 2 × 16 = 32 and hydrogen 2 × 1 = 2 (1 mark). Mr = 40 + 32 + 2 = 74 (1 mark).
Example 3
Calculate the relative formula mass of H₂SO₄. Ar: H = 1, S = 32, O = 16. [2 marks]
Model answer
Hydrogen contributes 2 × 1 = 2, sulfur 32, and oxygen 4 × 16 = 64 (1 mark). Mr = 2 + 32 + 64 = 98 (1 mark).
Common mistakes
Ignoring the subscript outside a bracket.
It multiplies every atom inside the bracket, not just the last one.
Using the atomic number instead of the relative atomic mass.
Use the larger of the two numbers shown in the periodic table.
Adding instead of multiplying for repeated atoms.
Multiply the atomic mass by the number of atoms.
Giving Mr units.
Relative formula mass has no units.
Exam tips
- Write out each element's contribution separately before adding.
- Expand brackets carefully before counting atoms.
- Use the larger periodic table number for relative atomic mass.
- Check reactant and product totals match in a balanced equation.
Key terms
- Relative formula mass
- The sum of the relative atomic masses in a formula.
- Relative atomic mass
- The average mass of an atom compared with carbon-12.
- Subscript
- The small number showing how many atoms are present.
- Formula
- A representation of the atoms present in a compound.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.