In Liebig's method for the estimation of C and `H`, if the compound also contains N, which of the following is kept near the exist of the combustion tube?

Options

A. silver wire
B. `PbCrO_4`
C. Both (a) and (b)
D. `Cu` gauge

Explanation

**Step-by-Step Solution:**

1. **Understanding Liebig's Method**: Liebig's method is used for the quantitative analysis of carbon (C) and hydrogen (H) in organic compounds. The organic compound is combusted in the presence of pure oxygen.

**Hint**: Remember that Liebig's method is primarily for estimating C and H in organic compounds.

2. **Combustion Process**: During the combustion of the organic compound, carbon is oxidized to carbon dioxide (CO2) and hydrogen is oxidized to water (H2O).

**Hint**: Identify the products formed during combustion: CO2 and H2O.

3. **Absorption of Products**: The CO2 produced is absorbed by a solution like potassium hydroxide (KOH) or sodium hydroxide (NaOH), while the water vapor is absorbed by anhydrous calcium chloride (CaCl2).

**Hint**: Know the absorbents used for CO2 and H2O in the process.

4. **Presence of Nitrogen**: If the organic compound contains nitrogen (N), during combustion, nitrogen can also be oxidized to nitrogen oxides (NO or NO2).

**Hint**: Consider what happens to nitrogen during combustion.

5. **Absorption of Nitrogen Oxides**: The nitrogen oxides produced will also be absorbed by the KOH solution along with CO2.

**Hint**: Recognize that KOH absorbs both CO2 and nitrogen oxides.

6. **Role of Copper Gauge**: To ensure that the nitrogen oxides are converted to nitrogen gas (N2) before they exit the combustion tube, a copper gauge is placed near the exit of the combustion tube. The copper reduces the nitrogen oxides to nitrogen gas.

**Hint**: Understand the function of the copper gauge in reducing nitrogen oxides.

7. **Conclusion**: Therefore, in Liebig's method for the estimation of C and H, if the compound contains N, a copper gauge is kept near the exit of the combustion tube to facilitate the reduction of nitrogen oxides.

**Final Answer**: Copper gauge.