Under what pressure must an equimolar mixture of `C_2` and `PCl_3` be placed at `250^@C` in order to obtain 80% conversion of `PCl_3` into `PCl_5? K_p` for
`PCl_3(g) + Cl_2(g) iff PCl_5 (g)` is 1.78

Explanation

To solve the problem, we need to determine the pressure at which an equimolar mixture of \( PCl_3 \) and \( Cl_2 \) must be placed at \( 250^\circ C \) to achieve 80% conversion of \( PCl_3 \) into \( PCl_5 \). The equilibrium constant \( K_p \) for the reaction is given as 1.78.

### Step-by-Step Solution:

1. **Write the balanced chemical equation:**
\[ PCl_3(g) + Cl_2(g) \rightleftharpoons PCl_5(g) \]
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