Relative formula mass (Mr)
The relative formula mass (Mr) is the mass of a whole formula — you get it by adding up the Ar of every atom in it. It's the closest thing GCSE Chemistry has to a free two marks, it comes up nearly every year, and it's also the opening step of half the harder calculations (moles, reacting masses, percentage by mass, percentage yield).
How to do it — step by step
- Count how many of each atom are in the formula — the little subscript number applies to the atom just before it.
- Multiply each atom's Ar by how many there are.
- Add it all up — never multiply the Ar values together. No units.
- Show the substitution line ("56 × 2 + 16 × 3") — it's a mark on its own, and it protects you if the arithmetic slips.
Worked examples — from real AQA papers
Where students lose marks — from the examiner reports
- Ignoring the subscripts. Adding each Ar once (1 + 12 + 16 for H₂CO₃) — flagged in both the 2022 and 2024 reports. On the 2024 CO₂ question, fewer than a third of Combined Foundation students could do it.
- Multiplying instead of adding. "56 × 112" (2023), multiplying the Ar values together (2022, 2024) — Mr is always a sum.
- Reading subscripts backwards. Fe₂O₃ done as three Fe + two O (2023).
- Subtracting the wrong way in reverse questions (2023) — subtract the known groups from the total, not from each other.
- No working shown. An arithmetic slip with the substitution written down still earns a mark; the same slip alone earns nothing (2023 report).
How often it comes up on AQA papers
From Science Street's database of real AQA questions:
| Paper | Marks | What you had to do |
|---|---|---|
| Chemistry Paper 1 Foundation 2020 | 2 | Mr of FeBr₃ → 296 |
| Chemistry Paper 1 Foundation 2021 | 2 | Reverse: 3H₂ + RO₃ = 150, find Ar of R → 96 |
| Chemistry Paper 1 Foundation 2022 | 2 | Mr of H₂CO₃ → 62 |
| Combined Science Chemistry Paper 1 Foundation 2023 | 2 | Mr of H₂SO₄ → 98 |
| Chemistry Paper 1 Foundation 2023 | 2 | Mr of Fe₂O₃ → 160 |
| Chemistry Paper 2 Higher 2023 | 2 | Reverse: amino acid section from Mr = 75 → 14 |
| Combined Science Chemistry Paper 2 Foundation 2024 | 2 | Mr of CO₂ → 44 |
The pattern: a near-annual 2-marker, mostly Foundation, mostly Paper 1 — one mark for the substitution, one for the answer. Higher tier sees it hidden inside bigger calculations instead, which is exactly why it's worth making automatic: get Mr wrong at step one of a 5-marker and every number after it is wrong too.
Now practise it — free
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