Which of the following statements is/are correct?

Options

A. The conjugate acid of `NH_(2)^(-)` is `NH_(3)`
B. Solubility product constant increases with increase in concentration of ions
C. On diluting a buffer solution pH change is negligible
D. In alkaline buffer solution, if some HCl is added, its `[OH^(-)]` will increase

Answer

To determine which statements are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: The conjugate acid of amidine is ammonia.**
- Amidine has the structure R-C(=NH)-NH2. The conjugate acid is formed when a base accepts a proton (H+).
- The amide ion (NH2-) can accept a proton to form ammonia (NH3).
- Therefore, this statement is **correct**.

2. **Statement 2: The solubility product constant increases with the increase in concentration of ions.**
- The solubility product constant (Ksp) is a constant at a given temperature for a sparingly soluble salt and does not change with the concentration of the ions in solution.
- Ksp is defined as the product of the molar concentrations of the ions, each raised to the power of their coefficients in the balanced equation.
- Therefore, this statement is **incorrect**.

3. **Statement 3: On diluting a buffer solution, pH change is negligible.**
- A buffer solution is designed to resist changes in pH upon the addition of small amounts of acids or bases.
- When a buffer solution is diluted, the ratio of the concentrations of the weak acid and its conjugate base remains the same, thus the pH remains relatively unchanged.
- Therefore, this statement is **correct**.

4. **Statement 4: In an alkaline buffer solution, if some HCl is added, its OH- concentration will increase.**
- An alkaline buffer consists of a weak base and its conjugate acid. When HCl (a strong acid) is added, it increases the concentration of H+ ions.
- The added H+ ions will react with the weak base present in the buffer, forming more of the conjugate acid and consuming OH- ions.
- Thus, the concentration of OH- will actually **decrease**, making this statement **incorrect**.

### Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Incorrect
- Statement 3: Correct
- Statement 4: Incorrect

Thus, the correct options are **1 and 3**.