Relative formula mass

Simple molecules have very small masses. Instead of writing their actual masses in grams or kilograms, we usually use their relative formula masses. These are calculated using the chemical formula and the relative atomic masses of the elements in it.

Relative formula mass

Relative atomic mass has the symbol Ar. The Ar for an element is a measure of the mass of its atoms compared to the mass of carbon-12 atoms. The greater the Ar value, the more mass an element's atoms have. The periodic table shows the Ar value for each element. For example, the Ar for carbon is 12, and the Ar for magnesium is 24. This means that a magnesium atom has twice the mass of a carbon atom.

Calculating relative formula mass

Relative formula mass is given 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

Ar and 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. The formulae used are their empirical formulae.

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