Relative formula mass

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

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

Calculating relative formula 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, Mr. 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

For example, the formula for carbon dioxide is CO2. It consists of one carbon atom (Ar = 12) and two oxygen atoms (Ar = 16):

Mr of CO2 = 12 + 16 + 16 = 44

It could also be calculated this way:

Mr of CO2 = (1 × 12) + (2 × 16) = 12 + 32 = 44

Like Ar values, Mr values are just numbers. They have no units because they are relative masses.

Relative formula masses of ionic compounds

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.

Worked example

Question

Calculate the relative formula mass, Mr, of calcium hydroxide, Ca(OH)2.

(Ar of Ca = 40, Ar of O = 16, Ar of H = 1)

Mr = 40 + (2 × 16) + (2 × 1)

= 40 + 32 + 2

= 74

It could also be calculated this way:

Mr = 40 + 2 × (16 + 1)

40 + 34 = 74

Question

Calculate the relative formula mass, Mr, of magnesium nitrate, Mg(NO3)2.

(Ar of Mg = 24, Ar of N = 14, Ar of O = 16)

Mr = 24 + (2 × 14) + (2 × 3 × 16)

= 24 + 28 + 96

= 148

It could also be calculated this way:

Mr = 24 + 2 × [14 + (16 × 3)]

= 24 + 124

= 148