Answer
$$K_c = \frac{[Cl_2]}{[PCl_5]}$$
Work Step by Step
- Since $PCl_3$ is liquid in this reaction, its concentration is constant. Thus, it should not appear in the equilibrium expression.
- In order to fix this mistake, just remove $[PCl_3]$ from the expression.
$$K_c = \frac{[Cl_2]}{[PCl_5]}$$
