Vapour density = molar mass, M/2

To find the molar mass of air, use the relative abundance formula modified for molecules

Molar mass of air mixture, M_{a }= M_{N2} X Frac_{N2} + M_{O2} X Frac_{O2}

where

M_{N2} is molar mass of N_{2}= 28g/mol

Frac_{N2} is fractional abundance of N_{2} = 80% or 0.8

M_{O2} is molar mass of O_{2} = 32g/mol

Frac_{O2} is fractional abundance of O_{2} = 20% or 0.2

SUbstituting the values

M_{a} = 28 X 0.8 + 32 X 0.2

M_{a} = 22.4 + 6.4

M_{a} = 28.8g/mol

Vapour density of air = M_{a}/2

Vapour density of air = 28.8/2

Vapour density of air = 14.4