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.

Question

Find the Mr of carbon monoxide, CO.

The Ar of carbon, C = 12, and the Ar of oxygen, O = 16.

The Mr of carbon monoxide, CO = 12 + 16 = 28.

Question

Find the Mr of sodium oxide, Na2O.

The Ar of sodium, Na = 23, and the Ar of oxygen, O = 16.

The Mr of sodium oxide, Na2O = (23 × 2) + 16 = 62.

(Note, the workings could also be shown as 23 + 23 + 16)

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.

Relative formula mass calculations

The table shows some more examples of relative formula mass calculations, using the relative atomic mass values listed beneath it.

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 hydrogen, H = 1
  • Ar of oxygen, O = 16
  • Ar of sodium, Na = 23
  • Ar of magnesium, Mg = 24