At 250ºC, the equilibrium constant K_p for the reaction PCl₅(g) PCl₃(g)+ Cl₂(g)is 1.80. Sufficient PCl₅ is put into a reaction vessel to give an initial pressure of 2.74 atm at 250ºC. Calculate the pressure of PCl₅ after the system has reached equilibrium.

Question

Quizplus · Accepted Answer

We can use the equilibrium constant expression to set up an ICE table and solve for the equilibrium pressures of the gases.

PCl5(g) ⇌ PCl3(g) + Cl2(g)

Initial:   2.74 atm   0          0 
Change:   -x            +x        +x 
Equilibrium: 2.74-x   x          x

K = [PCl3][Cl2]/[PCl5] = (x)(x)/(2.74-x)

Substitute the equilibrium concentrations into the expression for K and solve for x:

1.80 = x^2/(2.74-x)
x = 0.961 atm

Therefore, the equilibrium pressures are:

PCl5 = 2.74 - x = 1.78 atm
PCl3 = x = 0.961 atm
Cl2 = x = 0.961 atm

The pressure of PCl5 at equilibrium is 1.78 atm, so the answer is B) 1.24 atm.

At 250ºC, the Equilibrium Constant Kp for the Reaction PCl5(g)

At 250ºC, the Equilibrium Constant K_p for the Reaction PCl₅(g)