Question

A 7.95-atm PCl₅ is initially placed in an empty vessel at 25˚C. Calculate K_p if the equilibrium partial pressure of Cl₂ is 3.00 atm at the same temperature.

I am completely lost on how to even start the problem, let alone solve it.

Answer 1

PCl5   <--------------------------> PCl3 + Cl2

7.95                                        0          0      ----------------------> initial pressure

-x                                           +x       +x     -----------------------> changed pressure

7.95 - x                                      x        x    ------------------------> equilibrium pressure

from data equilibrium pressure of Cl2 is given . that means x is given

x= 3 atm

now equilibirum pressure of all components

PCl5 partial pressure = 7.95 -x = 7.95 -3 = 4.95 atm

PCl3 partial pressure   = x = 3 atm

Cl2 partial pressure   = x = 3 atm

now equilibrium constant Kp

Kp = P_PCl3 x P_Cl2 / P_PCl5

Kp = 3 x 3 / 4.95

Kp = 1.82 -----------------------------(answer)