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Chemical calculations - both Tiers

Relative formula mass

For your exam, you will need to know what relative formula mass is - and be able to work out the relative formula mass of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] when given its formula.

To find the relative formula mass (Mr) of a compound, you just add together the Ar values for all the atoms in its formula.


  • Example 1: Find the Mr of carbon monoxide (CO).
  • The Ar of carbon is 12 and the Ar of oxygen is 16.
  • So the Mr of carbon monoxide is 12 + 16 = 28.
  • Example 2:Find the Mr of sodium oxide (Na2O).
  • The Ar of sodium is 23 and the Ar of oxygen is 16.
  • So the Mr of sodium oxide is (23 x 2) + 16 = 62.

The relative formula mass of a substance, shown in grams, is called one mole of that substance. So one mole of carbon monoxide has a mass of 28g, and one mole of sodium oxide has a mass of 62g.

The table shows some more examples of relative formula mass calculations, using the relative atomic mass values given at the bottom of the page.

Relative formula mass calculations

WaterH2O1 + 1 + 16 =18
Sodium hydroxideNaOH23 + 16 + 1 = 40
Magnesium hydroxideMg(OH)224 + 16 + 16 + 1 + 1 = (remember that there are two of each atom inside the brackets)58
  • Ar of H = 1
  • Ar of O = 16
  • Ar of Na = 23
  • Ar of Mg = 24

Back to Chemical calculations index

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