1. Given equilibrium

$\mathrm{PCl_5(g)} \rightleftharpoons \mathrm{PCl_3(g)} + \mathrm{Cl_2(g)}$

Here,

• Left side moles of gas $= 1$
• Right side moles of gas $= 2$

So,

$\Delta n = 2 - 1 = +1$

2. What happens when xenon is added at constant $T$ and $p$?

Xenon is an inert gas. At constant temperature and pressure, adding an inert gas increases the total volume of the system.

As volume increases, the partial pressures of all reacting gases decrease.

If all partial pressures are reduced by the same factor $k$ ($k<1$), then the reaction quotient becomes

$Q_p = \frac{P_{\mathrm{PCl_3}}\,P_{\mathrm{Cl_2}}}{P_{\mathrm{PCl_5}}}$

After dilution,

$Q_p' = \frac{(kP_{\mathrm{PCl_3}})(kP_{\mathrm{Cl_2}})}{kP_{\mathrm{PCl_5}}} = k\,Q_p$

Since initially at equilibrium, $Q_p = K_p$, we get

$Q_p' = kK_p < K_p$

Thus,

$Q_p' < K_p$

So the equilibrium shifts forward (towards more moles of gas) to increase $Q_p$.

3. Effect on concentrations of species

Forward shift means:

• $\mathrm{PCl_5}$ decreases
• $\mathrm{PCl_3}$ increases
• $\mathrm{Cl_2}$ increases

Hence,

• Option A is false
• Option B is true
• Option C is false
• Option D is false

4. Correct option

$\boxed{\text{B: } \mathrm{PCl_3} \text{ will increase}}$

5. Comparison with stored answer

Stored correct answer: B

My derived answer: B

They agree.