On addition of KF in the solution of HF :-

Options

A. dissociation of HF increases.
B. concentration of `H_(+)`ion increases.
C. concentration of `H_(+)`ion decreases.
D. concentration of `F^(-)` ion decreaseses.

Explanation

### Step-by-Step Solution

1. **Identify the Components**:
- We have hydrofluoric acid (HF) in solution, which dissociates into H⁺ and F⁻ ions:
\[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \]

2. **Addition of KF**:
- When potassium fluoride (KF) is added to the HF solution, it dissociates completely into K⁺ and F⁻ ions:
\[ \text{KF} \rightarrow \text{K}^+ + \text{F}^- \]

3. **Effect of Common Ion**:
- The addition of KF increases the concentration of F⁻ ions in the solution. This is known as the common ion effect, where the presence of a common ion (F⁻ in this case) affects the equilibrium of the weak acid (HF).

4. **Le Chatelier's Principle**:
- According to Le Chatelier's principle, if a system at equilibrium is disturbed by changing the concentration of one of the components, the system will shift in a direction that counteracts the change.
- In this case, the increase in F⁻ concentration will shift the equilibrium to the left (backward direction):
\[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \]
- This shift results in a decrease in the concentration of H⁺ ions.

5. **Conclusion**:
- Therefore, upon adding KF to the HF solution, the concentration of H⁺ ions decreases due to the common ion effect.

### Final Answer
The addition of KF in the solution of HF results in a decrease in the concentration of H⁺ ions.

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