The equilibrium , `PCl_5 (g) hArr PCl_3(g) +Cl_2(g)` is attatined at `25^@C` in a closed container and an inert gas He is introduced . Which of the following statements are correct ?

Options

A. a) Concentration of `PCI_5 and CI_2` are changed
B. b) More `CI_2` is formed
C. c) Concentration of `PCI_3` is reduced
D. d) Nothing happens

Explanation

To solve the question regarding the equilibrium reaction \( PCl_5 (g) \rightleftharpoons PCl_3 (g) + Cl_2 (g) \) at \( 25^\circ C \) in a closed container, we need to analyze the effect of introducing an inert gas, helium (He), into the system.

### Step-by-Step Solution:

1. **Understanding the Equilibrium Reaction**:
The equilibrium reaction given is:
\[ PCl_5 (g) \rightleftharpoons PCl_3 (g) + Cl_2 (g) \]
This means that \( PCl_5 \) can dissociate into \( PCl_3 \) and \( Cl_2 \), and the system reaches a state where the rates of the forward and reverse reactions are equal.

2. **Effect of Adding an Inert Gas**:
When an inert gas like helium is introduced to the system at constant volume, it does not react with any of the components in the equilibrium. The partial pressures of the gases involved in the equilibrium remain unchanged because the inert gas does not participate in the reaction.

3. **Analyzing the Options**:
- **Option A**: Concentration of \( PCl_5 \) and \( Cl_2 \) changed.
- This is incorrect. The concentrations of all species remain unchanged because the addition of an inert gas does not affect the equilibrium concentrations.

- **Option B**: More \( Cl_2 \) is formed.
- This is incorrect. The addition of helium does not shift the equilibrium position, so no additional \( Cl_2 \) is formed.

- **Option C**: Concentration of \( PCl_3 \) is reduced.
- This is incorrect. Similar to the previous points, the concentration of \( PCl_3 \) remains constant.

- **Option D**: Nothing happens.
- This is correct. The addition of an inert gas does not change the concentrations of the reactants or products at equilibrium.

4. **Conclusion**:
The correct statement is **Option D**: nothing happens. The concentrations of \( PCl_5 \), \( PCl_3 \), and \( Cl_2 \) remain unchanged when an inert gas is added to the system at equilibrium.