Given below are two statements: 

Assertion (A): In the dissociation of PCl5 at constant pressure and temperature, the addition of helium at equilibrium increases the dissociation of PCl5.
Reason (R): Helium reacts with Cl2 .
 
1. Both (A) and (R) are True and (R) is the correct explanation of (A).
2. Both (A) and (R) are True but (R) is not the correct explanation of (A).
3. (A) is True but (R) is False.
4. (A) is False but (R) is True.

Hint: At constant pressure, volume changes and concentration of reactant and product changes.
The reaction is as follows:
 
PCl5PCl3+Cl2
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.
Hence, Assertion is true but the reason is false.