Chapter 14, Problem 14.133QP

A mixture of 0.0565 mol phosphorus pentachloride, PCl₅, and 0.0800 mol helium gas, He, was placed in a 1.000-L flask and heated to 250.0°C. The phosphorus pentachloride decomposes at this temperature to give phosphorus trichloride, PCl₃, and chlorine gas, Cl₂. The helium gas is inert.

${\text{PCl}}_5(g)\stackrel{}{\rightleftharpoons }{\text{PCl}}_3(g)+{\text{Cl}}_2(g)$

What is the partial pressure of helium in this equilibrium mixture at 250.0°C? At equilibrium, the total pressure is found to be 6.505 atm. What is K_c for the dissociation of PCl₅?