50. Assertion (A) In the dissociation of PCl5 at constant pressure and temperature the addition of helium at equilbrium increases the dissociation of PCl5.

Reason (R) Helium removes Cl2 from the field at action.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not the correct explanation of A

(c) A is true but R is false

(d) Both A and R are false

(c) Assertion is true but reason is false
${\mathrm{PCl}}_{5}\to {\mathrm{PCl}}_{3}+{\mathrm{Cl}}_{2}$
At constant pressure, when helium is added to the equilibrium, volume increases. Thus in order to maintain the K constant, degree of dissociation of PCl5 increases. Helium is unreactive towards chlorine gas.