Relationship Between Vapour Density and Relative Molecular Mass

RELATIONSHIP BETWEEN VAPOUR DENSITY AND RELATIVE MOLECULAR MASS

1. 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 mass of one atom of Hydrogen.

2. Vapour Density:

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.

Vapour Density (V.D.) =

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, then

V.D. at S.T.P =

Cancelling 'n' which is common, you get V.D.

When you compare the formula of vapour density with relative molecular mass, they can be represented as V.D.

You can therefore substitute the above equation to an Eqn 7.1 and arrive at the following formula

Now on cross multiplication, you have

2 × vapour density = Relative molecular mass of a gas

(Or)

Relative molecular mass = 2 × Vapour density