Relative Molecular Mass: It is defined as the ratio of the mass of 1 molecule of the gas or vapour to the mass of 1 atom of hydrogen.
Vapour Density (V.D): It is defined as the ratio of the mass of a certain volume of the gas or vapour to the mass of the same volume of hydrogen at the same temperature and pressure.
Applying Avogadro’s Law,
Since hydrogen is diatomic,
2 x V.D = relative molecular mass of a gas or vapour
2 x Vapour density = Relative molecular mass
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