Calculating relative formula masses

Relative atomic mass

The relative atomic mass of an element shows its mass compared with the mass of atoms of other elements. The relative atomic mass of carbon is 12, while the relative atomic mass of magnesium is 24. This means that each magnesium atom is twice the mass of a carbon atom.

Relative formula mass

Relative atomic masses can be used to find the relative formula mass of a compound.

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

Here are two examples:

Question

Find the Mr of carbon monoxide, CO.

The Ar of carbon is 12 and the Ar of oxygen is 16.

The Mr of carbon monoxide is 12 + 16 = 28.

Question

Find the Mr of sodium oxide, Na2O.

The Ar of sodium is 23 and the Ar of oxygen is 16.

The Mr of sodium oxide is (23 × 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 28 g, and one mole of sodium oxide has a mass of 62 g.

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

CompoundFormulaCalculationRelative formula mass
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