Relative formula masses can be calculated and used in conservation of mass calculations. Calculations can be carried out to find out concentrations of solution and uncertainties in measurements.

Atoms have very little mass so their relative atomic masses are used. The relative atomic mass of an element, symbol *A*_{r}, is the relative mass of its atoms compared to the mass of a carbon-12 atom. The *A*_{r} values for elements are given in the periodic table. Since *A*_{r} is a measure of relative mass, it has no units.

See the Atomic structure study guide for more on calculating relative atomic mass.

The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Relative formula mass has the symbol, *M*_{r}. To calculate the *M*_{r} for a substance:

- work out how many atoms of each element there are in the chemical formula
- add together the
*A*_{r}values for all the atoms of each element present

For example, the formula for carbon dioxide is CO_{2}. It consists of one carbon atom (*A*_{r} = 12) and two oxygen atoms (*A*_{r} = 16):

*M*_{r} of CO_{2} = 12 + 16 + 16 = 44

It could also be calculated this way:

*M*_{r} of CO_{2} = (1 × 12) + (2 × 16) = 12 + 32 = 44

Like *A*_{r} values, *M*_{r} values are just numbers. They have no units because they are **relative** masses.

Ionic compounds such as sodium chloride do not exist as molecules. However, their relative formula masses are calculated in the same way, from the numbers shown in the formula.

- Question
Calculate the relative formula mass,

*M*_{r}, of calcium hydroxide, Ca(OH)_{2}.(

*A*_{r}of Ca = 40,*A*_{r}of O = 16,*A*_{r}of H = 1)*M*_{r}= 40 + (2 × 16) + (2 × 1)= 40 + 32 + 2

= 74

It could also be calculated this way:

*M*_{r}= 40 + 2 × (16 + 1)40 + 34 = 74

- Question
Calculate the relative formula mass,

*M*_{r}, of magnesium nitrate, Mg(NO_{3})_{2}.(

*A*_{r}of Mg = 24,*A*_{r}of N = 14,*A*_{r}of O = 16)*M*_{r}= 24 + (2 × 14) + (2 × 3 × 16)= 24 + 28 + 96

= 148

It could also be calculated this way:

*M*_{r}= 24 + 2 × [14 + (16 × 3)]= 24 + 124

= 148