A metal nitrate reacts with `KI` to give a black precipitate which on addition of excess of `Kl` is converted into orange colour solution.The cation of the metal nitrate is :

Options

A. `Hg^(2+)`
B. `Bi^(3+)`
C. `Pb^(2+)`
D. `Cu^(+)`

Answer

To solve the problem, we need to identify the cation of the metal nitrate that reacts with potassium iodide (KI) to form a black precipitate, which then turns into an orange solution upon the addition of excess KI.

### Step-by-Step Solution:

1. **Identify the Reaction with KI**:
- When a metal nitrate reacts with KI, we need to consider the possible cations that can form a black precipitate. The black precipitate indicates the formation of a specific compound with iodine.

2. **Evaluate the Cations**:
- We have four options to consider: Hg²⁺, Bi³⁺, Pb²⁺, and Cu⁺.

3. **Check Each Cation**:
- **Hg²⁺ (Mercury)**:
- When Hg²⁺ reacts with KI, it forms a yellow-green precipitate of Hg₂I₂. This does not match the black precipitate described in the question.
- **Bi³⁺ (Bismuth)**:
- When Bi³⁺ reacts with KI, it forms BiI₃, which is a black precipitate. Upon adding excess KI, it forms a complex compound KBi₄, which is orange in color. This matches the conditions given in the question.
- **Pb²⁺ (Lead)**:
- Pb²⁺ reacts with KI to form a yellow precipitate of PbI₂. This does not match the black precipitate described.
- **Cu⁺ (Copper)**:
- Cu⁺ reacts with KI to form a white precipitate of Cu₂I₂. This also does not match the black precipitate described.

4. **Conclusion**:
- The only cation that fits the description of forming a black precipitate (BiI₃) and then turning into an orange solution (KBi₄) upon the addition of excess KI is Bi³⁺. Therefore, the cation of the metal nitrate is **Bi³⁺**.

### Final Answer:
The cation of the metal nitrate is **Bi³⁺**.