Deduction of molecular formula from its volumetric composition

Molecular Formula of a substance gives the names and number of atoms of the various elements present and the molecular mass of it.

To find the molecular formula from percentage composition we have to undertake the following steps:

1) First find the empirical formula, as illustrated earlier.

2) Then find the empirical formula mass: for this we have to add the atomic masses, of all the atoms in the empirical formula.

For e.g., assume the empirical formula of a compound is HO.

Empirical formula mass = Mass of 1 hydrogen atom + mass of 1 oxygen atom

= 1 + 16 = 17

3) The relative molecular mass or molar mass is obtained from experiment or from the vapour density relationship: molecular mass = 2 x vapour density.

In the example cited above let the molar mass be = 34.

4) We then have to find the ratio (n) of the relative molecular mass to the empirical formula mass. This ratio is calculated as

From the example above 'n'

5) The molecular formula is found from the above ratio by using the realtion:

Molecular formula = n x empirical formula

In the example, the relative molecular mass is found to be two times greater than the empirical formula mass,

the molecular formula = empirical formula x 2

= (HO) x 2 = H₂O₂