Relative molecular mass (hydrogen scale):
The relative molecular mass of a gas or vapour is the ratio between the mass of one molecule of the gas or vapour to the mass of one atom of hydrogen.
Vapour density is the ratio of the mass of a certain volume of a gas or vapour, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure.
According to Avogadro's law, all gases have the same number of molecules in equal volumes.
Thus, let the number of molecules in one volume \(= n\), then
Cancelling '\(n\)' which is common, you get,
However, since hydrogen is a diatomic,
When you compare the formula for vapour density and relative molecular mass, you get,
Relative molecular mass (hydrogen scale)
By substituting the equation (\(2\)) into equation (\(1\)), we get,
Now, on cross multiplication, we have,