Here it is
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You remember how density and molar mass is directly proportional to each other in the ideal gas equation
PM= dRT
Vapor density is the ratio of the density of given gas with H2 gas
D1/D2 = M1/M2
M1 and D1 are the molar mass and density of the given gas
M 2 and D2 molar mass and density of the hydrogen gas

The density of hydrogen gas if taken as one then vapour density of any gas is  its molar mass divided by 2
So Vapour density = Molar mass of the gas /2

In the case of the second and third equation, there is a dimer formation which results in less experimental molar mass density the theoretical value.
Due to this the experimental vapour density is smaller than the theoretical value