For the reaction, ${\mathrm{PCl}}_{3\left(\mathrm{g}\right)}+{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}\to {\mathrm{PCl}}_{5\left(\mathrm{g}\right)}$ the position of equilibrium can be shifted to the right by

The chemical equilibrium is

${\mathrm{PCl}}_{3\left(\mathrm{g}\right)}+{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}\to {\mathrm{PCl}}_{5\left(\mathrm{g}\right)}$The position of equilibrium will shift towards right by the addition of ${\mathrm{Cl}}_2$at constant volume. If increase the concentration of reactants forward reaction favors more.