By which of the following reagents can a sublimate of `HgCl_(2)` be distinguished from `NH_(4)Cl` ?

Options

A. `H_(2)S`
B. `BaCl_(2)`
C. `NaNO_(3)`
D. `FeCl_(3)`

Explanation

To distinguish between a sublimate of HgCl₂ (mercury(II) chloride) and NH₄Cl (ammonium chloride), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Compounds**:
- We have two substances: HgCl₂ (mercury(II) chloride) and NH₄Cl (ammonium chloride).

2. **Consider the Reagents**:
- The reagents provided for the distinction are H₂S (hydrogen sulfide), barium chloride, sodium nitrate, and ferric chloride.

3. **Test with H₂S**:
- When H₂S is added to HgCl₂, a black precipitate of HgS (mercury sulfide) is formed, and hydrochloric acid (HCl) is released.
- The reaction can be represented as:
\[ \text{HgCl}_2 + \text{H}_2\text{S} \rightarrow \text{HgS (black ppt)} + 2\text{HCl} \]
- In contrast, NH₄Cl does not react with H₂S, meaning no precipitate will form.

4. **Conclusion from H₂S Test**:
- Since HgCl₂ produces a black precipitate with H₂S while NH₄Cl does not react, we can distinguish between the two substances using H₂S.

5. **Test with Other Reagents**:
- **Barium Chloride**: This reagent does not react with either HgCl₂ or NH₄Cl in a way that would allow for distinction.
- **Sodium Nitrate**: Similarly, sodium nitrate does not provide a distinguishing reaction with either compound.
- **Ferric Chloride**: Ferric chloride also does not react with either HgCl₂ or NH₄Cl to create a distinguishing characteristic.

6. **Final Answer**:
- The only reagent that allows us to distinguish between HgCl₂ and NH₄Cl is H₂S.

### Final Answer:
The reagent that can be used to distinguish a sublimate of HgCl₂ from NH₄Cl is **H₂S (hydrogen sulfide)**.