Calculate Relative Mass (Edexcel IGCSE Chemistry: Double Science)

How to calculate relative formula mass

• The symbol for the relative atomic mass is A_r
• This is calculated from the mass number and relative abundances of all the isotopes of a particular element
• The symbol for the relative formula mass is M_r and it refers to the total mass of the molecule
• To calculate the M_r of a substance, you have to add up the relative atomic masses of all the atoms present in the formula

Relative formula mass calculations table

Substance	Atoms present	Calculation	Mr
Hydrogen H2	2 x H	(2 x 1)	2
Water H2O	(2 x H) + (1 x O)	(2 x 1) + (1 x 16)	18
Potassium carbonate K2CO3	(2 x K) + (1 x C) + (3 x O)	(2 x 39) + (1 x 12) + (3 x 16)	138
Calcium hydroxide Ca(OH)2	(1 x Ca) + (2 x O) + (2 x H)	(1 x 40) + (2 x 16) + (2 x 1)	74
Ammonium sulfate (NH4)2SO4	(2 x N) + (8 x H) + (1 x S) + (4 x O)	(2 x 14) + (8 x 1) + (1 x 32) + (4 x 16)	132

• In accordance with the Law of Conservation of Mass, the sum of the relative formula masses of the reactants will be the same as the sum of the relative formula masses of the products