4 mole of ${\mathrm{PCl}}_5$ is heated in 1L vessel to establish the equilibrium $\left[PC{l}_{5\left(g\right)}\rightleftharpoons PC{l}_{3\left(g\right)}+C{l}_{2\left(g\right)}\right]$ At equilibrium 1 mole of ${\mathrm{Cl}}_2$ is formed. Then $K_c$ value for the above equilibrium is