When `H_(2)S` gas is passed through aqueous solution of `CuCl_(2), HgCl_(2), BiCl_(3)` and `CoCl_(2)` in the presence of excess of dilute HCl, it fails to precipitate

Options

A. `CuS`
B. `HgS`
C. `Bi_(2)S_(3)`
D. `CoS`

Explanation

To solve the question regarding the reaction of `H₂S` gas with aqueous solutions of `CuCl₂`, `HgCl₂`, `BiCl₃`, and `CoCl₂` in the presence of excess dilute `HCl`, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Group of Metals**:
- The metals present in the solutions are Copper (Cu), Mercury (Hg), Bismuth (Bi), and Cobalt (Co).
- Copper, Mercury, and Bismuth belong to Group 2 of the qualitative analysis scheme, while Cobalt belongs to Group 4.

2. **Understand the Role of `H₂S`**:
- `H₂S` is a reagent used to precipitate sulfides of certain metals. In acidic medium, it can precipitate sulfides of Group 2 metals.

3. **Reaction with `H₂S`**:
- When `H₂S` is passed through the solutions of `CuCl₂`, `HgCl₂`, and `BiCl₃`, the following reactions occur:
- `CuCl₂ + H₂S → CuS (s) + 2HCl`
- `HgCl₂ + H₂S → HgS (s) + 2HCl`
- `BiCl₃ + H₂S → Bi₂S₃ (s) + 6HCl`
- All these reactions produce precipitates (CuS, HgS, and Bi₂S₃).

4. **Cobalt's Reaction**:
- Cobalt (Co) from `CoCl₂` does not precipitate with `H₂S` in the presence of dilute `HCl`.
- Cobalt belongs to Group 4, and to precipitate cobalt sulfide (CoS), a basic medium (like `NH₄OH`) is required.

5. **Conclusion**:
- In the presence of excess dilute `HCl`, the `H₂S` fails to precipitate CoS, while it successfully precipitates the sulfides of Cu, Hg, and Bi.

### Final Answer:
- The metal that fails to precipitate when `H₂S` is passed through the solution is **Cobalt (Co)**.