Question

At 250 °C, the equilibrium constant Kp 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 250 °C. Calculate the pressure of PCl5 after the system has reached equilibrium.
a. 1.50 atm
b. 1.24 atm
c. 4.24 atm
d. 0.94 atm
e. 1.12 atm

Answer 1

Answer: b. 1.24 atm

Explanation:

The chemical reaction for the decomposition of phosgene follows the equation:

                  $PCl_5(g)\rightleftharpoons PCl_3(g)+Cl_2(g)$

At t = 0         2.74         0     0    

At $t=t_{eq}$    2.74-x           x      x

The expression for $K_p$ for the given reaction follows:

$K_p=\frac{p_{PCl_3}\times p_{Cl_2}}{p_{PCl_5}}$

Putting values in above equation, we get:

$1.80=\frac{x\times x}{2.74-x}$

$x=1.50$

So, the partial pressure for the components at equilibrium are:

$p_{PCl_5}=2.74-x=2.74-1.50=1.24atm$

Hence, the partial pressure of the $PCl_5$ at equilibrium is 1.24 atm