Assertion: In the dissociation of PCl5 at constant pressure and temperature, the addition of helium at equilibrium increases the dissociation of PCl5.

Reason: Helium reacts with Cl2 .

1.  Both the assertion and the reason are true and the reason is the correct explanation of assertion.

2.  Both the assertion and the reason are true and the reason is not the correct explanation of assertion.

3.  Assertion is true but the reason is false.

4.  Assertion is false but the reason is true.

Hint: At constant pressure, volume changes and concentration of reactant and product changes.
The reaction is as follows:

${\mathrm{PCl}}_{5}\to {\mathrm{PCl}}_{3}+{\mathrm{Cl}}_{2}$
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.
Hence, Assertion is true but the reason is false.