Establish the relationship between vapour density and molecular mass of a gas by applying Avogadro’s law.
As we know, Vapor density is defined as “mass of a certain volume of a gas divided by the mass of the same volume of hydrogen”.
Avogadro’s law which states that “At same temperature and pressure, equal volume of all gases will have the same number of molecules”.
V.D. = Mass of 1 volume of gas or vapourMass of 1 volume of hydrogen
Applying Avogadro’s law,
V.D. = Mass of 1 molecule of gas or vapourMass of 1 molecule of hydrogen
Since hydrogen is diatomic,
V.D .= Mass of 1 molecule of gas or vapour2× mass of 1 atom of hydrogen
2 × V.D. = Relative molecular mass of gas or vapour
Therefore relative molecular mass = 2 × Vapour density