# Relative formula mass

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

### Calculating relative formula mass

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

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

To calculate the Mr for a substance:

1. work out how many atoms of each element there are in the chemical formula
2. add together the Ar 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)

Mr of CO2 = 12.0 + 16.0 + 16.0 = 44.0

It could also be calculated this way:

Mr 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)

Mr 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:

Mr of Cl2 = (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)

Mr = 24.3 + (2 × 16.0) + (2 × 1.0)

= 24.3 + 32.0 + 2.0

= 58.3

It could also be calculated this way:

Mr = 24.3 + 2 × (16.0 + 1.0)

= 24.3 + 34.0 = 58.3

Ar and Mr values are numbers with no units. Ar values are often shown in periodic tables and, if needed, are given in exam questions.