The reagents like `AgNO_(3),K_(4)[fe(CN)_(6)],KCNS,KI,K_(2)CrO_(4)` Nessler's ect, find extenstive and very important applicasion in quentitative analysis because these resgents form different type of produce with different cation for axample `KI` form yellow precipitate with `Pb^(2+)` but at form red iprecipitate with `Hg^(2+)` , Hence these reagents are widely used in the quantitative amalysis of morgenic salts
Which of the following cation (i.e. basic radicals) forms coloured (not white) percipitates with aqueous solution of potassium iodide the precipitete does not dissolve is exiess og reagent?

Options

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

Explanation

To solve the question, we need to determine which cation forms a colored precipitate with potassium iodide (KI) that does not dissolve in excess reagent. Let's analyze the given options step by step.

### Step 1: Analyze the Cations
We have four cations to consider:
1. Hg²⁺ (Mercuric ion)
2. Hg₂²⁺ (Mercurous ion)
3. Bi³⁺ (Bismuth ion)
4. Cu²⁺ (Copper ion)

### Step 2: Reaction with Potassium Iodide (KI)
- **Hg²⁺ (Mercuric ion)**:
- When reacted with KI, it forms a red or scarlet precipitate of HgI₂.
- However, this precipitate dissolves in excess iodide ions to form a soluble complex, HgI₄²⁻.
- **Conclusion**: Does not satisfy the condition.

- **Hg₂²⁺ (Mercurous ion)**:
- When reacted with KI, it forms a green precipitate of HgI₂.
- In excess iodide, it does not dissolve but instead forms HgI₄²⁻ and a black precipitate of elemental mercury (Hg).
- **Conclusion**: Satisfies the condition of forming a colored precipitate that does not dissolve in excess reagent.

- **Bi³⁺ (Bismuth ion)**:
- When reacted with KI, it forms a black precipitate of BiI₃.
- This precipitate dissolves in excess iodide to form a soluble complex, BiI₄⁻.
- **Conclusion**: Does not satisfy the condition.

- **Cu²⁺ (Copper ion)**:
- When reacted with KI, it forms a white precipitate of CuI.
- **Conclusion**: Does not satisfy the condition as it must be colored.

### Step 3: Conclusion
Based on the analysis, the only cation that forms a colored precipitate with potassium iodide and does not dissolve in excess reagent is the mercurous ion, Hg₂²⁺.

### Final Answer
The cation that forms a colored (not white) precipitate with aqueous potassium iodide that does not dissolve in excess reagent is **Hg₂²⁺ (Mercurous ion)**.

---