Calculate Relative Formula Mass (AQA GCSE Chemistry: Combined 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 molecular masses of the reactants will be the same as the sum of the relative molecular masses of the products

• The percentage by mass of an element in a compound can be calculated using the following equation:

• For example, in water:
  • Water is a simple molecule with the chemical formula H₂O
  • So, water is made of two hydrogen (H) atoms and one oxygen (O) atom
  • From the Periodic Table, the relative atomic mass of:
    • Hydrogen = 1
    • Oxygen = 16
  • Therefore, the total mass of water is:
    • (2 × 1) + 16 = 18
• To find the percentage by mass of hydrogen:
  • Percentage of hydrogen = × 100 = 11.1%
• Similarly, the percentage by mass of oxygen is:
  • Percentage of oxygen = × 100 = 88.9%
• Note: The total percentage by mass of all the elements should add up to 100%, e.g. 11.1% + 89.9% = 100%