Why does the dissociation of `PCl_5` decrease in the presence of `Cl_2` ?

Explanation

To understand why the dissociation of \( PCl_5 \) decreases in the presence of \( Cl_2 \), we can analyze the reaction and apply Le Chatelier's Principle step by step.

### Step 1: Write the dissociation reaction of \( PCl_5 \)

The dissociation of phosphorus pentachloride (\( PCl_5 \)) can be represented by the following chemical equation:

\[
PCl_5 (g) \rightleftharpoons PCl_3 (g) + Cl_2 (g)
...