In the dissociation of PCl5 asPCl5(g)⇌PCl3(g)+Cl2(g)if the degree of dissociation is α (alpha) at equilibrium pressure P, then the equilibrium constant for the reaction is

PCl5     ⇌     PCl3      +      Cl2 Initial                      1                     0                     0 Final                     1-α                 α                     α  Partial  pressure      =1−α1+α⋅P   α  1+α⋅P   α1+α⋅P  Total mole =1−α+α+α=1+α Kp=α1+αP⋅α1+α⋅P1−α1+αP=α2P1−α2