1.33 calculate empirical and molecular formulae from experimental data

Calculating Molecular Formula

1Write the element symbolsSO
Write the masses or the % of the elements 3.2 g6.4 g – 3.2 g = 3.2 g
Write the Ar values of each element3216
Divide masses by At 
3.2 ÷ 32 = 0.1 3.2 ÷ 16 = 0.2
5Divide by the smallest number0.1 ÷ 0.1 = 1 0.2 ÷ 0.1 = 2
6Write the formulaSO2


Calculating Molecular Formula
From the empirical formula, you can work out the molecular formula from the relative formula mass (Mr) of the compound.
  • Add up the atomic masses of the atoms in the empirical formula.
For example, the empirical formula of a hydrocarbon is CH2 and its Mr is 42.
  • The mass of the atoms in the empirical formula is 14
  • 42 ÷ 14 = 3
  • so you need to multiply the numbers in the empirical formula by 3
The molecular formula of the hydrocarbon is therefore C3H6.


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