Suppose the reaction PCl5(s)  ⇌  PCl3(g)  +   Cl2(g)  is in a closed vessel at equilibrium stage. What is the effect on equilibrium concentration of Cl2(g) by adding PCl5 at constant temperature ?

On adding more PCl5, equilibrium shifts forward.∵According to lechatelier's principle increase in the concentration of reactants (or addition of reactants) and decrease in the concentration(or removal) of products favours the forward reaction.