Two organic compounds ‘X’ and ‘Y’ containing carbon and hydrogen only have vapour densities 13 and 39 respectively. State the molecular formula of ‘X’ and ‘Y’ [C = 12, H = 1]

Vapour density_X = 13

Vapour density_Y = 39

Molecular weight_X = 2 × V.D. = 2 x 13 = 26

Molecular weight_Y = 2 × V.D. = 2 x 39 = 78

Let formula for X be C_aH_b

where a and b are simple whole numbers.

∴ Molecular mass of X = 12a + b = 26

Hence, a = 2 and b = 2 (only values which hold true for the above equation)

So, Molecular formula of X = C₂H₂

Let formula for Y be C_cH_d

where c and d are simple whole numbers.

∴ Molecular mass of Y = 12c + d = 78

Hence, c = 6 and d = 6 (only values which hold true for the above equation)

So, Molecular formula of Y = C₆H₆