Which of the following substances cannot be used for drying `NH_3`

Options

A. Anhyd `CaCl_2`
B. `P_2O_5`
C. Conc `H_2SO_4`
D. CaO

Answer

To determine which substances cannot be used for drying ammonia (NH₃), we need to analyze the properties of the drying agents typically used and their interactions with ammonia.

### Step-by-Step Solution:

1. **Understanding Drying Agents**:
Drying agents are substances that absorb moisture from gases or liquids. Common drying agents include calcium oxide (CaO), magnesium sulfate (MgSO₄), and phosphorus pentoxide (P₂O₅).

2. **Calcium Oxide (CaO)**:
Calcium oxide, also known as quicklime, is a strong drying agent. It can effectively absorb water and is commonly used for drying gases, including ammonia.

3. **Magnesium Sulfate (MgSO₄)**:
Magnesium sulfate is another effective drying agent. It can absorb moisture and is used for drying various gases, including ammonia.

4. **Phosphorus Pentoxide (P₂O₅)**:
Phosphorus pentoxide is one of the most powerful drying agents and can absorb water vapor from gases, making it suitable for drying ammonia.

5. **Identifying the Incorrect Drying Agent**:
In the context of the question, we need to identify a substance that cannot be used for drying ammonia. Some substances, like sodium hydroxide (NaOH) or silica gel, can react with ammonia or may not be effective in drying it.

6. **Conclusion**:
Based on the properties of the substances mentioned, if one of the options provided is sodium hydroxide (NaOH) or another substance that reacts with ammonia, it would be the correct answer to the question.

### Final Answer:
The substance that cannot be used for drying ammonia is sodium hydroxide (NaOH) or any other substance that reacts with ammonia.

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