The answer to each of the following is a non-negative integer.
`H_2S` gas can be dried by how many of the following reagents?
Anhydrous `CaCl_2` conc `H_2SO_2P_2O_5` KOH solution, `Na_2CO_3` solution

Answer

To determine how many of the given reagents can be used to dry `H2S` gas, we need to analyze each reagent based on its properties as a drying agent.

### Step 1: Identify the Reagents
The reagents listed are:
1. Anhydrous `CaCl2`
2. Concentrated `H2SO4`
3. `P2O5`
4. KOH solution
5. `Na2CO3` solution

### Step 2: Analyze Each Reagent
- **Anhydrous `CaCl2`:** This is a well-known drying agent that can absorb moisture from gases. It can effectively dry `H2S`.

- **Concentrated `H2SO4`:** This is not just a drying agent but also a dehydrating agent. It can remove water by chemical reaction rather than just physical absorption. However, it can still be considered for drying purposes.

- **`P2O5`:** Phosphorus pentoxide is a powerful dehydrating agent and can effectively remove water from gases, including `H2S`.

- **KOH solution:** Potassium hydroxide is a strong base and can absorb moisture but is primarily used for chemical reactions rather than as a drying agent for gases. It can react with `H2S`, which makes it less suitable for drying.

- **`Na2CO3` solution:** Sodium carbonate can absorb moisture but is not typically used as a drying agent for gases. It also has the potential to react with `H2S`.

### Step 3: Count the Suitable Drying Agents
From the analysis:
1. Anhydrous `CaCl2` - Yes
2. Concentrated `H2SO4` - Yes
3. `P2O5` - Yes
4. KOH solution - No
5. `Na2CO3` solution - No

Thus, the total number of reagents that can be used to dry `H2S` is **3**.

### Final Answer
The answer is **3**.

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