Arrange the following compound in decreasing order of their basicity.
`(a)HO^(-)" "(b)NH_(3)" "(c)H_(2)O`

Answer

To arrange the compounds \( \text{HO}^- \), \( \text{NH}_3 \), and \( \text{H}_2\text{O} \) in decreasing order of their basicity, we need to analyze the basicity of each compound based on their ability to accept protons (H\(^+\)).

### Step-by-Step Solution:

1. **Understanding Basicity**: Basicity is defined as the ability of a compound to accept protons. The more readily a compound can accept a proton, the stronger the base it is.

2. **Analyzing Each Compound**:
- **(a) \( \text{HO}^- \)**: This is the hydroxide ion. It has a negative charge, which indicates a strong tendency to accept protons. The presence of the negative charge enhances its basicity significantly.
- **(b) \( \text{NH}_3 \)**: Ammonia has a lone pair of electrons on the nitrogen atom, which can be used to accept a proton. Therefore, it is a good base, but it is less basic than the hydroxide ion due to the absence of a negative charge.
- **(c) \( \text{H}_2\text{O} \)**: Water is a neutral molecule and has a lower tendency to accept protons compared to \( \text{HO}^- \) and \( \text{NH}_3 \). Its basicity is limited because it is more electronegative and less inclined to donate its electrons.

3. **Comparing Basicity**:
- \( \text{HO}^- \) (hydroxide ion) is the strongest base due to its negative charge.
- \( \text{NH}_3 \) (ammonia) is the next strongest base because of its lone pair of electrons.
- \( \text{H}_2\text{O} \) (water) is the weakest base as it is neutral and less inclined to accept protons.

4. **Final Order**: Based on the analysis above, we can arrange the compounds in decreasing order of their basicity:
\[ \text{HO}^- > \text{NH}_3 > \text{H}_2\text{O} \]