Relative Molecular Mass
Relative molecular mass is the ratio of the mass of one molecule of a substance to the mass th of a carbon atom, or 1 amu.
Vapour density is the ratio of the mass of a volume of a gas, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure.
According to Avogadro's law, equal volumes of all gases contain equal number of molecules.Thus, let the number of molecules in one volume = n,
Cancelling 'n' which is common, we get
However, since hydrogen is diatomic.
When we compare the formulae of vapour density with relative molecular molecular mass, they can be represented as
We can therefore substitute the above equation, and arrive at the below formula
Now on cross multiplication, we have2 x vapour density = Relative Molecular Mass of a gas
Relative Molecular Mass = 2 x Vapour density