If `PCl_5` is heated to `250^@C` and allowed to reach equilibrium, 50% of `PCl_5` is dissociated. Calculate how many moles of `Cl_2` must be mixed with one mole of `PCl_5` to reduce the dissociation to 40%, volume remaining constant.

Explanation

To solve the problem, we need to analyze the dissociation of \( PCl_5 \) and how the addition of \( Cl_2 \) affects the equilibrium.

### Step 1: Write the dissociation reaction
The dissociation of \( PCl_5 \) can be represented as:
\[ PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) \]

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