Atoms have very little mass. Instead of using their actual masses in kilograms, we use their relative atomic masses. Relative atomic mass is given the symbol A_r. The A_r of an element is the mean mass of its atoms compared to 1/12^th the mass of a carbon-12 atom.

Calculating relative formula mass

Relative formula mass is given the symbol M_r. If the substance consists of molecules, the term relative molecular mass can be used instead.

The M_r of a substance is the mean mass of one of its units compared to 1/12^th the mass of a carbon-12 atom.

To calculate the M_r for a substance:

1. work out how many atoms of each element there are in the chemical formula
2. add together the A_r values for all the atoms of each element present

Example 1

Calculate the relative formula mass of carbon dioxide, CO₂.

(Relative atomic masses: C = 12.0, O = 16.0)

M _r of CO₂ = 12.0 + 16.0 + 16.0 = 44.0

It could also be calculated this way:

M _r of CO₂ = (1 × 12.0) + (2 × 16.0) = 12.0 + 32.0 = 44.0

Example 2

Calculate the relative formula mass of chlorine, Cl₂.

(Relative atomic mass: Cl = 35.5)

M _r of Cl₂ = 35.5 + 35.5 = 71.0

Note that the answer is given to 1 decimal place. It could also be calculated this way:

M _r of Cl₂ = (2 × 35.5) = 71.0