A-Level Chemistry: Empirical and Molecular Formulaa



  • The empirical formula of a compound is the formula which shows the simplest whole-number ratio in which the atoms in that compound exist.
  • It can be calculated if the composition by mass of the compound is known.
  • The molecular formula of a substance is the formula which shows the number of each type of atom in the one molecule of that substance.
  • It applies only to molecular substances, and can be deduced if the empirical formula and molar mass of the compound are known.
  • The molecular formula is always a simple whole number multiple of the empirical formula.
  • Eg a substance contains 85.8% carbon and 14.2% hydrogen, what is its empirical formula? If its relative molecular mass is 56, what is its molecular formula?

It is also possible to calculate the percentage composition by mass of a substance, if its empirical or molecular formula is known.

Eg What is the percentage composition by mass of ethanoic acid, C2H4O2?

RMM = 60

% C = (12 x 2)/60 x 100 = 40.0%

% H = (1 x 4)/60 x 100 = 6.67%

%O = (16 x 2)/60 x 100 = 53.3%