Molecular and empirical formulae

The empirical formula gives the simplest whole number ratio of the atoms in a substance. 

 In the actual formula for a substance the number of each type of atom that is present may be a multiple of that indicated by the empirical formula.  

The actual molar mass of a substance can be determined by titration or using mass spectrometry.  If the experimentally-determined molar mass of the molecule is different from the total mass of atoms in the empirical formula, the empirical formula is NOT the actual formula of the substance. 

For example hydrogen peroxide has the empirical formula HO. 

For OH, the total mass of the atoms is 17 g mol–1
The experimentally-determined molar mass of hydrogen peroxide is 34 g mol–1

The actual formula for the substance can be determined if both the molar mass and the empirical formula are known.
 
  1. Divide the molar mass by the total mass of the empirical formula to obtain the number by which all subscripts in the empirical formula must be multipled.
  2. Multiply all atoms in the empirical formula by this factor to deduce the molecular formula.
For hydrogen peroxide this factor is 2, and the molecular formula is H2O2.
The actual structure of H2O2 is HO–OH.
Dr Sheila Woodgate
© The University of Auckland 2002