Relative Molecular Mass – IGCSE Chemistry Definition
IGCSE Chemistry definition of relative molecular mass (Mr): the sum of relative atomic masses in a molecular formula. Covers calculation method and examples.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Relative molecular mass (Mr) is the sum of all the relative atomic masses in a molecule’s formula. It is used constantly in mole calculations and stoichiometry on the 0620 syllabus. Getting Mr right is often the first step in an exam calculation, and an error here cascades through the entire answer.
The 0620 definition
The relative molecular mass (Mr) of a substance is the sum of the relative atomic masses of all the atoms shown in the molecular formula, compared to 1/12 of the mass of an atom of carbon-12.
For ionic compounds (which do not form molecules), the same calculation gives the relative formula mass, but it is calculated in exactly the same way.
How to calculate Mr
- Write out the formula
- Count the number of each type of atom
- Multiply each by its Ar value
- Add them all together
Worked examples
(Ar values: H = 1, C = 12, N = 14, O = 16, S = 32, Ca = 40, Na = 23, Cl = 35.5)
| Substance | Formula | Calculation | Mr |
|---|---|---|---|
| Water | H₂O | 2(1) + 16 | 18 |
| Carbon dioxide | CO₂ | 12 + 2(16) | 44 |
| Sulfuric acid | H₂SO₄ | 2(1) + 32 + 4(16) | 98 |
| Calcium carbonate | CaCO₃ | 40 + 12 + 3(16) | 100 |
| Sodium hydroxide | NaOH | 23 + 16 + 1 | 40 |
| Calcium hydroxide | Ca(OH)₂ | 40 + 2(16 + 1) | 74 |
Brackets in formulae
When a formula contains brackets, multiply everything inside by the subscript outside.
Ca(OH)₂: the subscript 2 applies to both O and H inside the brackets.
- Ca: 1 x 40 = 40
- O: 2 x 16 = 32
- H: 2 x 1 = 2
- Mr = 74
Mg(NO₃)₂: the subscript 2 applies to everything inside.
- Mg: 1 x 24 = 24
- N: 2 x 14 = 28
- O: 6 x 16 = 96 (3 oxygens inside brackets x 2)
- Mr = 148
Worked exam question
Calculate the relative formula mass of aluminium sulfate, Al₂(SO₄)₃. (Ar: Al = 27, S = 32, O = 16) (2)
Mark scheme
Al: 2 x 27 = 54; S: 3 x 32 = 96; O: 12 x 16 = 192 [1]; Mr = 54 + 96 + 192 = 342 [1]
Common exam mistakes
- Forgetting to multiply inside brackets by the subscript. In Ca(OH)₂, there are 2 oxygens and 2 hydrogens, not 1 of each.
- Using mass number instead of Ar from the periodic table. Mass number is for a specific isotope; Ar is the weighted average shown on the periodic table.
- Giving Mr with units. Like Ar, relative molecular mass has no units.
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