${\mathrm{PCl}}_5\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$

In given equilibrium reaction, volume of container is inereased by mixing inert gas at constant temperature :

equilibrium will shift in forward direction and concentration of ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$ will decrease 

equilibrium will shift in backward direction and concentration of ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$ will decrease

equilibrium will shift in forward direction and concentration of ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$ will increase 

equilibrium will shift in backward direction and concentration of ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$ will increase