Example 1 - Molecular Formula

The percentage composition of a compound is 92.26% C and 7.74% H. The relative molecular mass of the compound is 78. Determine its molecular formula of the compound

Answer:
In order to find molecular formula of a chemical compound, we need to find the empirical formula of the compound before hand.

Finding the empirical formula:
Element
C
H
Percentage
92.26%
7.74%
Mass in 100g
92.26g
7.74g
Number of mole
92.26 12 =7.688 mol
7.74 1 =7.74 mol
Simple ratio
1
1

The empirical formula of the compound = CH

Finding the molecular formula 

Let's say the molecular formula of the compound
= CnHn

where n is a round number.

The relative molecular mass of  CnHn
= n(12) + 4(1) = 78
13n = 78
n = 78/13 = 6

The molecular formula of the compound
C6H6