Yellow colour of `HNO_3` is due to the presence of `NO_2` is removed by

Options

A. Boiling the acid
B. Adding `Mg` powder
C. Passing `NH_3` through acid
D. Passing air through warn acid.

Answer

To solve the question regarding the yellow color of HNO3 (nitric acid) being due to the presence of NO2 (nitrogen dioxide) and how to remove it, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Cause of Yellow Color**:
- The yellow color of HNO3 is primarily due to the presence of nitrogen dioxide (NO2), which is a product of the thermal decomposition of nitric acid.

2. **Identify the Reaction**:
- When nitric acid (HNO3) undergoes thermal decomposition, it produces nitrogen dioxide (NO2), water (H2O), and oxygen (O2). The reaction can be represented as:
\[ 4 \text{HNO}_3 \rightarrow 4 \text{NO}_2 + 2 \text{H}_2\text{O} + \text{O}_2 \]
- This reaction explains why NO2 is present in the solution, contributing to its yellow color.

3. **Determine Methods to Remove NO2**:
- To remove the yellow color (NO2) from HNO3, we need to find a method that effectively drives out NO2 from the solution.

4. **Evaluate the Options**:
- **Boiling the acid**: This may not effectively remove NO2 as it can also lead to further decomposition.
- **Adding magnesium (Mg) powder**: This is not a suitable method for removing NO2.
- **Passing ammonia (NH3) through the acid**: This does not directly address the removal of NO2.
- **Passing air through warm acid**: This method can help in driving out NO2, as the air can carry away the gas, leading to a colorless solution.

5. **Conclusion**:
- The correct method to remove the yellow color from HNO3 due to NO2 is by **passing air through the warm acid**. Thus, the correct answer is option 4.

### Final Answer:
The yellow color of HNO3 is due to the presence of NO2, which can be removed by **passing air through the warm acid**.