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	$$\mathrm{92.26g}$$	$$\mathrm{7.74g}$$
Number of mole	$$\frac{92.26}{12}=7.688\text{ mol}$$	$$\frac{7.74}{1}=7.74\text{ mol}$$
Simple ratio	$$1$$	$$1$$

The empirical formula of the compound = CH

Finding the molecular formula 
Let's say the molecular formula of the compound
= C_nH_n

where n is a round number.

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

The molecular formula of the compound
= C₆H₆