1.16 calculate relative formula masses (Mr) from relative atomic masses (Ar)

  1. Find the atomic mass number (Number of protons and neutrons) 
  2. If there is multiple of the molecule, for example O2, the atomic mass number must be multiplied, e.g. ( 16 ( Ar) x 2) = 32
  3. Add up elements if it’s a compund = RFM (Mr) 

Worked example : Potassium Sulphate (compound) – K2SO4

  1. Potassium Ar = (39 x 2) = 78 
  2. Sulfur Ar = 32
  3. Oxygen AR = (16 x 4) = 64
  4. 78 + 32 + 64 = 174
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