As per Lechatlier's Principle any stress applied on the equilibrium state is minimised by shifting of equilibrium.
`PCl_(5) hArr PCl_(3) + Cl_(2) " , " SO_(2) Cl_(2) hArr SO_(2) + Cl_(2)`
Both equilibria exist together in a flask. If some `SO_(2)` is introduced into the flask ______

Options

A. `SO_(2) Cl_(2)` concentration decreases
B. degree of dissociation of `PCI_(5)` decreases
C. degree of dissociation of `PCI_(5)` increases
D. `PCl_(3)` concentration decreases

Answer

To solve the problem, we will analyze the effects of adding \( SO_2 \) to the equilibrium systems based on Le Chatelier's Principle.

### Step-by-Step Solution:

1. **Identify the Equilibria**:
We have two equilibria in the flask:
- Reaction 1: \( PCl_5 \rightleftharpoons PCl_3 + Cl_2 \)
- Reaction 2: \( SOCl_2 \rightleftharpoons SO_2 + Cl_2 \)

2. **Effect of Adding \( SO_2 \)**:
When \( SO_2 \) is introduced into the flask, we need to consider how it affects the second equilibrium:
- For the second equilibrium \( SOCl_2 \rightleftharpoons SO_2 + Cl_2 \), adding \( SO_2 \) will increase the concentration of \( SO_2 \).

3. **Applying Le Chatelier's Principle**:
According to Le Chatelier's Principle, if a stress (in this case, an increase in \( SO_2 \)) is applied to a system at equilibrium, the system will shift in the direction that reduces that stress.
- Since \( SO_2 \) concentration is increased, the equilibrium will shift to the left to decrease the concentration of \( SO_2 \):
\[ SO_2 + Cl_2 \rightleftharpoons SOCl_2 \]

4. **Consequences of the Shift**:
- As the equilibrium shifts to the left, the concentration of \( Cl_2 \) will decrease because it is being consumed to form \( SOCl_2 \).
- The decrease in \( Cl_2 \) will also affect the first equilibrium \( PCl_5 \rightleftharpoons PCl_3 + Cl_2 \). Since \( Cl_2 \) is decreasing, the first equilibrium will shift to the left as well, leading to:
- A decrease in the concentration of \( PCl_3 \)
- An increase in the concentration of \( PCl_5 \)

5. **Conclusion**:
- The addition of \( SO_2 \) leads to a decrease in the concentration of \( Cl_2 \), which in turn decreases the concentration of \( PCl_3 \) and increases the concentration of \( PCl_5 \).

### Summary of Changes:
- **Decrease in \( Cl_2 \)**: Due to the shift in the second equilibrium.
- **Decrease in \( PCl_3 \)**: Due to the shift in the first equilibrium.
- **Increase in \( PCl_5 \)**: Due to the shift in the first equilibrium.

### Final Answer:
- The correct options based on the analysis are:
- The concentration of \( SOCl_2 \) decreases.
- The degree of dissociation of \( PCl_5 \) increases.