Calculate the equilibrium partial pressures of the reactants and products if the initial pressures are PPCl5 = 0.4200 atm and PPCl3 = 0.4950 atm.

Question

For the reaction below, K_p = 25.5 at 800 K.

PCl5 (g) --> PCl3 (g) + Cl2 (g)

Calculate the equilibrium partial pressures of the reactants and products if the initial pressures are P_PCl5 = 0.4200 atm and P_PCl3 = 0.4950  atm.

Answer 1

PCl5 (g) --> PCl3 (g) + Cl2 (g)

0.4200............0.4950.......0............Initial

-x...................+x...........+x..........Change

0.4200-x.........0.4950+x....x...........Equilibrium

Kp = (PCl3)(Cl2) / (PCl5)

25.5 = (0.4950-x)(x) / (0.4200-x)

10.71 - 25.5x = 0.4950x - x²

x² - 25.99x + 10.71 = 0

x = 0.4188 atm

At equilibrium, partial pressures are as follows;

(PCl5) = 0.4200 - 0.4188 = 1.2x10^-3 atm

(PCl3) = 0.4950 + 0.4188 = 0.9138 atm

(PCl5) = 0.4188 atm