Empirical formula in chemistry refers to a smallest integer ratio of the atoms present in a compound. For example, ethane has molecular formula C2H6. The ratio of number of C atoms to the number of H atoms is 2:6. The simplest integer ratio would be 1:3 and, thus, the empirical formula of ethane is CH3. Empirical formula can be deduced from either the mass or the percentage of each element present in a compound.

As an example, let us calculate the empirical formula of a compound containing elements carbon, hydrogen and oxygen. The percentages by mass of C and H in the compound is 66.67% and 11.11% respectively. The relative atomic masses for C = 12, H = 1, O = 16.

First, we notice that we have not been given the percentage of oxygen. However, since the percentage must add up to 100% we can work out the percentage of oxygen as equal to (100.00 – 66.67 – 11.11 = 22.22%).

Now, we work out the number of moles of each element using the formula:

number of moles = mass or percentage/relative atomic mass.

Elements                     Carbon                           Hydrogen                          Oxygen

% by mass                    66.66                                   11.11                                22.22

moles                          66.66/12                              11.11/1                          22.22/16

=                                     5.555                                   11.11                              1.38875

Now, divide by the smallest mole

The simplest  ratio          4                                         8                                      1

Thus,  the empirical formula of the compound is C4H8O.