Two organic compounds X and Y contain Carbon and Hydrogen only and have vapour densities 13 and 39 respectively. State the molecular formula of X and Y.

Molecular mass = 2 × vapour density
Given vapour density of x =13
Molecular mass = 2 ×13
= 26
The only organic compound containing Carbon and Hydrogen with molecular weight - 26 is C₂H₂ (Acetylene). [Since 2×12 + 2×1 = 26]


{Since atomic mass of Carbon=12 and Hydrogen=1}


Given vapour density of y = 39
Molecular mass = 2 × 39
= 78
The only organic compound containing Carbon and Hydrogen with molecular weight - 78 is C₆H₆ (Benzene). [Since 6×12 + 6×1 = 78]