The experimental values of the vapour density of either `NH_(4)Cl or PC l_(5)` is less than that obtained from the equation `D=M//s`. Explain.

Explanation

At the experimental temperature, both `NH_(4)Cl and PCl_(5)` undergo thermal dissociation.
`NH_(4)Cl (vap) hArr NH_(3)(g)+HCl (g)`
`PCl_(5) (vap) hArr PCl_(5)(g)+Cl_(2)(g)`
Due to such dissociation, number of molecules in `NH_(4)Cl` vapour or `PCl_(5)` vapour increases. this causes ann increase in volume because at a certain temperature and pressure, the volume of a gas is proportional to the number of molecules. as the mass is fixed, increase in volume causes decrease in the value of vapour density measured experimentally.