Which of the following gases can be dried by conc. `H_2SO_4` ?

Options

A. HCl
B. HBr
C. HI
D. `H_2S`

Answer

To determine which of the given gases can be dried by concentrated sulfuric acid (H₂SO₄), we need to analyze the reactivity of each gas with concentrated H₂SO₄.

### Step-by-Step Solution:

1. **Identify the Gases**: The gases in question are HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), and H₂S (hydrogen sulfide).

2. **Understand the Drying Process**: Concentrated H₂SO₄ is a strong dehydrating agent. For a gas to be dried by concentrated H₂SO₄, it should not react with it.

3. **Analyze Each Gas**:
- **HCl**: Hydrochloric acid is a strong acid but does not react with concentrated H₂SO₄. Therefore, it can be dried using concentrated H₂SO₄.
- **HBr**: Hydrobromic acid can react with concentrated H₂SO₄, leading to oxidation reactions. Thus, it cannot be dried by concentrated H₂SO₄.
- **HI**: Hydroiodic acid also reacts with concentrated H₂SO₄, resulting in oxidation. Therefore, it cannot be dried by concentrated H₂SO₄.
- **H₂S**: Hydrogen sulfide can react with concentrated H₂SO₄, resulting in the formation of sulfur dioxide (SO₂) and water. Hence, it cannot be dried by concentrated H₂SO₄.

4. **Conclusion**: Among the gases listed, only HCl can be dried using concentrated H₂SO₄ as it does not undergo any reaction with it.

### Final Answer:
The gas that can be dried by concentrated H₂SO₄ is **HCl**.