Relative Formula Mass – IGCSE Chemistry Definition and Key Facts
IGCSE Chemistry definition of relative formula mass: the sum of relative atomic masses of all atoms in a formula unit. Used for ionic compounds and mole calculations.
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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 formula mass is a fundamental calculation skill in the Cambridge 0620 syllabus. Every mole calculation, percentage composition, and empirical formula question depends on getting this number right.
The 0620 definition
The relative formula mass (symbol Mr) of a substance is the sum of the relative atomic masses (Ar) of all the atoms in its formula.
Relative formula mass has no units — it is a ratio compared to one-twelfth the mass of a carbon-12 atom.
Why “formula mass” not “molecular mass”?
Ionic compounds such as NaCl do not contain molecules — they consist of ions arranged in a giant ionic structure. Strictly, it is incorrect to say “relative molecular mass of NaCl.” The term relative formula mass applies to all compounds. However, at IGCSE level, the symbol Mr and the calculation method are the same regardless of which term is used.
How to calculate Mr
- Write out the formula
- Count the number of each type of atom (watch for brackets and subscripts)
- Multiply each count by the Ar of that element
- Add them all together
Examples
| Compound | Formula | Calculation | Mr |
|---|---|---|---|
| Water | H₂O | (2 x 1) + 16 | 18 |
| Carbon dioxide | CO₂ | 12 + (2 x 16) | 44 |
| Sodium chloride | NaCl | 23 + 35.5 | 58.5 |
| Calcium carbonate | CaCO₃ | 40 + 12 + (3 x 16) | 100 |
| Sulfuric acid | H₂SO₄ | (2 x 1) + 32 + (4 x 16) | 98 |
| Magnesium hydroxide | Mg(OH)₂ | 24 + 2 x (16 + 1) | 58 |
| Calcium nitrate | Ca(NO₃)₂ | 40 + 2 x (14 + 48) | 164 |
Dealing with brackets
Brackets in a formula mean everything inside is multiplied by the subscript outside.
Mg(OH)₂ means: 1 Mg, 2 O, 2 H
Ca(NO₃)₂ means: 1 Ca, 2 N, 6 O
Al₂(SO₄)₃ means: 2 Al, 3 S, 12 O
Mr of Al₂(SO₄)₃ = (2 x 27) + 3 x (32 + 4 x 16) = 54 + 3 x 96 = 54 + 288 = 342
Using Mr in mole calculations
The relative formula mass links mass to moles:
n = m / Mr
Where n = number of moles, m = mass in grams, Mr = relative formula mass.
Example: How many moles in 10 g of CaCO₃?
n = 10 / 100 = 0.10 mol
Rearranged: m = n x Mr (to find mass from moles)
Using Mr for percentage composition
Percentage of an element = (total Ar of that element in the formula / Mr) x 100
Example: % of calcium in CaCO₃ = (40 / 100) x 100 = 40%
Hydrated salts
For hydrated salts, include the water of crystallisation in the calculation.
CuSO₄.5H₂O: Mr = 64 + 32 + 64 + 5 x (2 + 16) = 160 + 90 = 250
Worked exam question
Calculate the relative formula mass of aluminium oxide, Al₂O₃. (Ar: Al = 27, O = 16) [1]
Mr = (2 x 27) + (3 x 16) = 54 + 48 = 102 [1]
Calculate the number of moles in 20.4 g of Al₂O₃. [1]
n = 20.4 / 102 = 0.20 mol [1]
Common exam mistakes
- Forgetting to multiply atoms inside brackets by the subscript outside. Mg(OH)₂ has 2 oxygen atoms and 2 hydrogen atoms, not 1 of each.
- Using the wrong Ar values. Always use the values given in the question or from the periodic table — do not round Cl to 36 (it is 35.5).
- Forgetting that Mr has no units. It is a dimensionless ratio.
- Confusing Mr with molar mass. Mr is a dimensionless number; molar mass is the mass of one mole and has units of g/mol. Numerically they are the same, but the units differ.
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