Question

If the empirical formula of two compounds is CH and their vapour densities are 13 to 39 respectively, find their molecular formula.

Answer 1

For acetylene , molecular mass = 2 × V.D = 2 × 13 = 26 g

The empirical mass = 12(C) + 1(H) = 13 g

n = `"Molecular formula mass"/"Empirical formula weight" = 26/13 = 2`

Molecular formula of acetylene = 2 × Empirical formula = C₂H₂

Similarly, for benzene molecular mass = 2 × V.D = 2 × 39 = 78

n = `78/13` = 6

So, the molecular formula = C₆H₆