A mixture is known to contain `NO_(3)^(-)` and `NO_(2)^(-)` before performing ring test for `NO_(3)^(-)`. The aq. Solution should be made free of `NO_(2)^(-)`. This is done by heating with

Options

A. `CO(NH_(2))_(2)`
B. `Zn` dust
C. Conc. `HNO_(3)`
D. dil. `HNO_(3)`

Answer

To solve the problem of removing `NO2^(-)` from a mixture containing both `NO3^(-)` and `NO2^(-)` before performing the ring test for `NO3^(-)`, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Interfering Ion**:
- We have a mixture containing `NO3^(-)` (nitrate ion) and `NO2^(-)` (nitrite ion). The nitrite ion is the interfering ion that needs to be removed before conducting the ring test for nitrate.

2. **Understand the Ring Test**:
- The ring test (also known as the brown ring test) is specifically used to detect the presence of `NO3^(-)` ions. The presence of `NO2^(-)` can interfere with this test.

3. **Choose a Suitable Reagent**:
- To remove `NO2^(-)`, we need a reagent that can react with it and eliminate it from the solution. Among the given options, urea is known to react with nitrous acid (`HNO2`), which is formed from `NO2^(-)`.

4. **Chemical Reaction**:
- The reaction between urea and nitrous acid is as follows:
\[ \text{HNO2} + \text{(CO(NH2)2)} \rightarrow \text{CO2} + \text{N2} + \text{H2O} \]
- In this reaction, urea reacts with nitrous acid to produce carbon dioxide, nitrogen gas, and water, effectively removing the nitrite ion from the solution.

5. **Conclusion**:
- Therefore, to make the aqueous solution free of `NO2^(-)` before performing the ring test for `NO3^(-)`, we should heat the solution with **urea**.

### Final Answer:
- The aqueous solution should be made free of `NO2^(-)` by heating with **urea**.