Assertion : In the dissociation of `PCl_(5)` at constant pressure and temperature addition of helium at equilibrium increases the dissociation of `PCl_(5)`.
Reason : Helium reacts with `Cl_(2)` and hence shifts the equilibrium in forward direction.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze the assertion and reason provided.

### Step 1: Understand the Assertion
The assertion states that the addition of helium at equilibrium increases the dissociation of \( PCl_5 \).

- **Dissociation Reaction**:
\[ PCl_5 (g) \rightleftharpoons PCl_3 (g) + Cl_2 (g) \]
- At equilibrium, the dissociation of \( PCl_5 \) can be influenced by changes in pressure or concentration.

### Step 2: Apply Le Chatelier's Principle
According to Le Chatelier's Principle, if a system at equilibrium is subjected to a change in conditions (like pressure or concentration), the equilibrium will shift in a direction that counteracts that change.

- **Effect of Adding Helium**:
- Helium is an inert gas and does not react with \( PCl_5 \), \( PCl_3 \), or \( Cl_2 \).
- When helium is added at constant pressure, the total volume of the gas mixture increases. This results in a decrease in the partial pressures of the reactants and products.

### Step 3: Analyze the Moles of Gas
In the dissociation of \( PCl_5 \):
- 1 mole of \( PCl_5 \) produces 1 mole of \( PCl_3 \) and 1 mole of \( Cl_2 \), resulting in a total of 2 moles of gas.
- Since the number of moles increases from 1 to 2, the equilibrium will shift to the right (forward direction) to produce more \( PCl_3 \) and \( Cl_2 \) when the concentration of the reactants decreases.

### Step 4: Conclusion on Assertion
Thus, the assertion is true: adding helium increases the dissociation of \( PCl_5 \) because it shifts the equilibrium to the right.

### Step 5: Evaluate the Reason
The reason states that helium reacts with \( Cl_2 \) and shifts the equilibrium in the forward direction.

- **Truth of the Reason**:
- This statement is false because helium is an inert gas and does not react with \( Cl_2 \) or any other species in this equilibrium.

### Final Conclusion
- **Assertion**: True
- **Reason**: False

Therefore, the correct answer is that the assertion is true, but the reason is false.