Question

A compound of carbon,hydrogen and nitrogen contains these elements in the ratio 9 : 1 : 3.5.If its molecular mass is 108,what is the molecular formula?

Answer 1

Given, the ratio of carbon, hydrogen and nitrogen in the compound is 9 : 1 : 3.5.

Therefore, in 100 g of compound,

Mass of carbon = 9 g

Mass of hydrogen = 1 g

Mass of nitrogen = 3.5 g

Moles of carbon = 9/12 = 0.75

Moles of hydrogen = 1/1 = 1

Moles of nitrogen = 3.5/14 = 0.25

Ratio of moles of carbon, hydrogen and nitrogen = 0.75 : 1 : 0.25

= 3 : 4 : 1

Empirical formula of the compound = C₃H₄N

Empirical formula mass = 12 x 3 + 4 x 1 + 14 x 1

= 54 u

Molecular mass = 108 u

n = 108/54

= 2

Therefore, molecular formula = (empirical formula) ₂

= (C₃H₄N)₂

= C₆H₈N₂