Hello Abhishek,

Vapour density is the density of vapours of a substance relative to the density of Hydrogen gas of same volume.

Since Density =Mass/Volume

And volume of hydrogen and the substance we are taking is same, Vapour Density =Molar mass Mass of substance X/ Molar mass of Hydrogen

Also molar mass of hydrogen is a fixed value(2.016)

Therefore Vapour Density of a substance= Molar Mass/2