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Calculating the Relative Formula Mass

Science: KS3, topic 2.6.6

Calculating the Relative Formula Mass (Mr)

To calculate the relative formula mass of a compound, we need to know the relative atomic masses (Ar) of the elements that make the compound. Then we multiply the Ar values by the number of atoms of that element in the compound. Then we add these numbers together.

E.g. hydrochloric acid (HCl)

E.g. hydrochloric acid (HCl)

  • Identify the relative atomic mass (Ar) of each element in the compound:
  • Ar of hydrogen = 1
  • Ar of chlorine = 35.5
    • So, the total Mr of HCl is 1 + 35.5 = 36.5
E.g. sulfuric acid (H2SO4)

E.g. sulfuric acid (H2SO4)

  • Identify the relative atomic mass (Ar) of each element in the compound:
  • Ar of hydrogen = 1
  • Ar of sulfur = 32
  • Ar of oxygen = 16
    • So, the total Mr of H2SO4 is (2x1) + (1x32) + (4x16) = 98

Test yourself on Calculating the Relative Formula Mass

  • What is the total Mr of H2O? (Ar of H=1, O=16)

  • What are the steps when calculating the relative formula mass of a compound?

  • Relative Formula Mass

  • Choose the correct statement

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