Chemists use symbols and formulae to represent elements, ions and compounds. Chemical equations model the changes that happen in chemical reactions.

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.

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:

- 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

**Example 1**

Calculate the relative formula mass of carbon dioxide, CO_{2}.

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

*M*_{r} of CO_{2} = 12.0 + 16.0 + 16.0 = 44.0

It could also be calculated this way:

*M*_{r} of CO_{2} = (1 × 12.0) + (2 × 16.0) = 12.0 + 32.0 = 44.0

**Example 2**

Calculate the relative formula mass of chlorine, Cl_{2}.

(Relative atomic mass: Cl = 35.5)

*M*_{r} of Cl_{2} = 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 Cl_{2} = (2 × 35.5) = 71.0

- Question
Calculate the relative formula mass of magnesium hydroxide, Mg(OH)

_{2}.(Relative atomic masses: Mg = 24.3, O = 16.0, H = 1.0)

*M*_{r}= 24.3 + (2 × 16.0) + (2 × 1.0)= 24.3 + 32.0 + 2.0

= 58.3

It could also be calculated this way:

*M*_{r}= 24.3 + 2 × (16.0 + 1.0)= 24.3 + 34.0 = 58.3