GCSE Chemistry (AQA)

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Relative Formula Mass

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Quantitative chemistry Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

Relative Formula Mass

9:12 Conservation and Changes of Mass in Chemical Reactions
Spec 4.3.1.2
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  • Relative atomic mass (AR) of an element can be found using the periodic table.
  • The AR is the larger of the two numbers given for an element.
  • Relative formula mass (MR) of a molecule or compound is the sum of the ARs of the atoms in the formula.
  • Example: To calculate MR of magnesium oxide (MgO), add AR of magnesium (24) to oxygen (16) to get 40.
  • Important to count the total number of each element in a molecule or compound.
  • Example: Potassium sulfate (K2SO4) has three elements: potassium, sulfur, and oxygen.
  • Potassium has AR of 39, sulfur 32, and oxygen 16.
  • Multiply AR by the number of atoms indicated by subscripts in the formula.
  • Example: For K2SO4, multiply potassium's AR by 2, sulfur's AR by 1, and oxygen's AR by 4.
  • Total MR for K2SO4 is 174.
  • Example question: Calculate MR of lithium sulfate (Li2SO4).
  • Lithium has AR of 7, sulfur 32, and oxygen 16.
  • Multiply AR by the number of atoms: lithium by 2, sulfur by 1, oxygen by 4.
  • Total MR for Li2SO4 is 110.
  • For compounds with compound ions, calculate mass of compound ions first, then multiply by subscripts.
  • Example: Magnesium hydroxide (Mg(OH)2) has magnesium AR of 24, hydroxide ion MR of 17.
  • Multiply hydroxide ion MR by 2 for total MR of 58.
  • Example: Magnesium nitrate (Mg(NO3)2) has magnesium AR of 24, nitrate ion MR of 62.
  • Multiply nitrate ion MR by 2 for total MR of 148.

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