A vessel contains 1 mole PCl5 (g) at 4 atm and 0.5 mole PCl3 formed at equilibrium. Now, equilibrium pressure of mixture is (assume ideal behavior)

p ∞n (V,T are constant)initial moles = 1 mole at equilibrium total moles = (1-0.5)+0.5+0.5 = 1.5pinitialpfinal =ninitial nfinal 4pf=11.5 Equilibrium pressure of the mixture  = pf =1.5×4 = 6 atm