Let the degree of dissociation be α.
P be the total pressure at equilibrium.
PCl5⇌PCl3+Cl2
Initial moles 1 0 0
At equi. 1−α α α
Total number of moles at equilibrium = 1−α+α+α = 1+α
Now, we know Kp = PPCl3×PCl2PCl5
PPCl5 = 1−α1+αP
PPCl3 = α1+αP
PCl2 = α1+αP
So, Kp = α1+αP×α1+αP1−α1+αP
Kp = α21−α2P
Given, P = 1 atm, α = 0.2
Kp=0.042 atm