The chemical formula of a compound can be determined from the composition of the compound. The composition of a compound is commonly expressed in terms of percentage of each element present in it.

To determine the formula of an unknown compound, follow the following steps,

Step 1. Determine the percentage of each element present in the compound from the mass of each element present in a certain known mass of the compound.

Step2. If the sum of percentages of all these elements is not 100, then the difference gives the percentage of oxygen. Calculate the percentage of oxygen in the given compound by using the following relationship.


Percentage of oxygen = 100- (Sum of the percentages of all other elements present in the compound)

Step 3: Divide the percentage of each element by the atomic mass of the respective element. the ratio so obtained is called atomic ratio.

Step 4: Divide the atomic ratios by the lowest value, and convert these into the nearest whole numbers. These whole numbers give the simplest ratio between the number of atoms of the various elements present in the compound.

Step 5: write down the empirical formula of the compound.


Step 6: Calculate the empirical formula mass by adding the atomic masses of all atoms present in the empirical formula.

Step 7: Obtain molecular mass ( or molar mass) either from experiment or from the vapour density of the compound by using the relationship.

Molecular mass = 2× Vapor density

Step 8. Obtain the value of n through the relation,


n = Molecular mass / Empirical formula mass

‘n’ is an integer such as 1,2 etc.

Step 9: The molecular formula is then obtained by

Molecular formula = n × Empirical formula