The best indicator for titrating HCl against `NH_(4)OH` is :

Options

A. Litmus
B. Phenolphthalein
C. Phenol red
D. Methyl orange

Explanation

To determine the best indicator for titrating hydrochloric acid (HCl) against ammonium hydroxide (NH₄OH), we can follow these steps:

### Step 1: Identify the Nature of the Acid and Base
- HCl is a strong acid, while NH₄OH is a weak base.

### Step 2: Understand the Reaction
- When HCl reacts with NH₄OH, it forms ammonium chloride (NH₄Cl) and water:
\[ \text{HCl} + \text{NH}_4\text{OH} \rightarrow \text{NH}_4\text{Cl} + \text{H}_2\text{O} \]

### Step 3: Analyze the pH at the Endpoint
- The resulting solution after the reaction contains ammonium chloride, which is a salt derived from a strong acid (HCl) and a weak base (NH₄OH).
- The hydrolysis of ammonium chloride will result in an acidic solution, leading to a pH that is less than 7.

### Step 4: Choose the Appropriate Indicator
- Indicators are chosen based on the pH range at which they change color.
- For a titration that results in an acidic solution, we need an indicator that changes color in the acidic pH range.

### Step 5: Evaluate Common Indicators
- **Methyl Orange**: Changes color from red to yellow between pH 3.1 to 4.5. This range is suitable for our titration since the endpoint will be in the acidic range.
- **Litmus**: Changes color around pH 4.5 to 8.3, which is not suitable for this titration.
- **Phenolphthalein**: Changes color from colorless to pink between pH 8.3 to 10, which is too high for this reaction.
- **Phenol Red**: Changes color from yellow to red between pH 6.2 to 8.2, which is also not suitable.

### Conclusion
- The best indicator for titrating HCl against NH₄OH is **Methyl Orange**, as it effectively indicates the endpoint in the acidic pH range.

### Final Answer
- **Methyl Orange** is the best indicator for this titration.

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