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Relative Formula MassOxford AQA IGCSE Chemistry: Revision notes

Section 1

What 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 shown in its chemical formula.

It tells you how heavy one 'unit' of the compound is compared with an atom of carbon-12, in the same way that Ar does for a single element.

For an element that exists as simple molecules (e.g. Cl2), the relative formula mass is used in exactly the same way as for a compound.

Key termsrelative formula mass (Mr)

Section 2

How do you calculate Mr from a formula?

  1. Identify every element in the formula and how many atoms of each are present
  2. Look up (or recall) the relative atomic mass (Ar) of each element
  3. Multiply each Ar by the number of atoms of that element
  4. Add all the values together
Example

Calculate the Mr of water, H2O (Ar: H = 1, O = 16). Mr = (2 × 1) + (1 × 16) = 2 + 16 = 18.

Section 3

How do you handle brackets in a formula?

When a formula contains brackets with a subscript outside them, such as Ca(OH)2, the subscript multiplies every atom inside the bracket.

Ca(OH)2 contains: 1 calcium atom, 2 oxygen atoms and 2 hydrogen atoms (not just 1 of each).

Common mistake

Forgetting to multiply every atom inside a bracket by the number outside it is one of the most common errors in Mr calculations — count each element carefully.

Example

Calculate the Mr of calcium hydroxide, Ca(OH)2 (Ar: Ca = 40, O = 16, H = 1). Mr = 40 + 2(16 + 1) = 40 + 34 = 74.

Section 4

How is Mr used in other calculations?

Relative formula mass is central to almost every quantitative chemistry calculation:

  • Converting between mass and moles: n = mass ÷ Mr
  • Calculating percentage composition of an element in a compound
  • Finding empirical and molecular formulae
  • Reacting mass calculations using balanced equations

Must Know

  • Mr = sum of the relative atomic masses (Ar) of all atoms shown in the formula
  • Multiply Ar by the number of atoms of each element, then add the totals together
  • Brackets multiply every atom inside them by the subscript outside
  • Mr has no units, just like Ar
  • Mr is essential for mole, percentage composition, empirical formula and reacting mass calculations

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