Relative Formula Mass — GCSE Chemistry Revision
Revise Relative Formula Mass for GCSE Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Conservation of MassWhat is Relative Formula Mass?
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance compared to the mass of a carbon-12 atom.
Board notes: Calculating relative formula mass is a fundamental skill required for many other calculations in chemistry, such as mole calculations. It is tested by all exam boards at all tiers.
Step-by-step explanationWorked example
To find the relative formula mass of magnesium carbonate (MgCO₃): Ar of Mg = 24, Ar of C = 12, Ar of O = 16. Mr(MgCO₃) = 24 + 12 + (3 x 16) = 24 + 12 + 48 = 84.
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Common mistakes
- 1Confusing relative atomic mass (Ar) with relative formula mass (Mr). Ar is for a single atom, while Mr is for a whole compound.
- 2Forgetting to multiply the Ar of an element by the number of atoms of that element in the formula (e.g., the '2' in H₂O).
- 3Making simple arithmetic errors when adding up the masses. Always double-check your calculations.
Relative Formula Mass exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Relative Formula Mass
Core concept
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance comp…
Frequently asked questions
What are the units for relative formula mass?
Relative formula mass is a ratio, so it has no units. It is just a number.
Where do I find the relative atomic masses?
Relative atomic masses for all elements are given in the periodic table, usually as the larger of the two numbers shown for each element.