Relationship between hydrolysis constant and dissociation constant are given. which is the correct formula to MgCl2 ?

A. Kh=KwKa

B. Kh=KwKb

C. Kh=KwKa×Kb

D. Kw=KhKb

To solve this question, let's first understand the relationship between hydrolysis constant \((K_h)\), dissociation constant \((K_a \text{ or } K_b)\), and water dissociation constant \((K_w)\).

### Step-by-Step Explanation:

1. **MgCl₂ Hydrolysis**:
- **Magnesium chloride (MgCl₂)** is a salt formed from a strong acid (HCl) and a moderately weak base (Mg(OH)₂).
- In aqueous solution, Mg²⁺ can undergo hydrolysis (react with water) to form Mg(OH)⁺ and release H⁺ ions, slightly acidifying the solution.

2. **Hydrolysis and Dissociation Constants**:
- The hydrolysis constant (\(K_h\)) of a salt is related to the dissociation constant of the weak part of the compound (in this case, Mg²⁺ acts like a weak base).
- \(K_a\) is the dissociation constant of an acid (H⁺ donor).
- \(K_b\) is the dissociation constant of a base (OH⁻ donor).
- \(K_w\) is the ionic product of water and is equal to \(10^{-14}\) at 25°C: \(K_w = [H^+][OH^-]\).

3. **Hydrolysis of Mg²⁺**:
- Since Mg²⁺ is a cation from a weak base (Mg(OH)₂), its hydrolysis constant should be related to \(K_b\).
- For a cation from a weak base, the correct formula relates the hydrolysis constant \(K_h\) to the basic dissociation constant \(K_b\).

4. **Formula Relationship**:
- The correct relationship for a cation hydrolyzing in water is:
\[
K_h = \frac{K_w}{K_b}
\]

### Conclusion:
For \( \text{MgCl}_2 \), the hydrolysis constant \( K_h \) is related to the basic dissociation constant \( K_b \), and the correct formula is:

\[
\boxed{K_h = \frac{K_w}{K_b}} \quad \text{(Option B)}
\]