4.0 moles of argon and 5.0 moles of $\mathrm{PCl}_5$ are introduced into an evacuated flask of 100 litre capacity at 610 K . The system is allowed to equilibrate. At equilibrium, the total pressure of mixture was found to be 6.0 atm . The Kp for the reaction is [Given: $\mathrm{R}=0.082 \mathrm{~L} \mathrm{~atm} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ ]