Relative formula mass

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 Ar. The Ar of an element 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 molecules, 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

curriculum-key-fact
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.