In Liebig's method for the estimation of C and H, if the compound also contains both halogens and S, which of the following is kept near the exit of the combustion tube?

Options

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

Explanation

To solve the question regarding Liebig's method for the estimation of carbon and hydrogen in the presence of halogens and sulfur, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Liebig's Method**:
Liebig's method is primarily used for the estimation of carbon (C) and hydrogen (H) in organic compounds. When the organic compound also contains halogens (X) and sulfur (S), additional steps are required to handle these elements during combustion.

**Hint**: Remember that Liebig's method focuses on the combustion of organic compounds to determine the amounts of carbon and hydrogen.

2. **Reactions with Halogens**:
During the combustion process, halogens present in the compound will react with silver (Ag). The silver wire is used to capture the halogen atoms, forming silver halide (AgX).

**Hint**: Consider how halogens behave in combustion and what materials are used to capture them.

3. **Formation of Silver Halide**:
The reaction can be represented as:
\[ \text{X} + \text{Ag} \rightarrow \text{AgX} \]
This silver halide (AgX) is formed and needs to be further processed.

**Hint**: Think about the products formed when halogens react with metals.

4. **Reactions with Lead Chromate**:
The silver halide (AgX) then reacts with lead chromate (PbCrO4) to form a layer of silver chromate (Ag2CrO4). This is important for quantifying the halogen content.

**Hint**: Identify the role of lead chromate in the process and what it reacts with.

5. **Placement of Materials**:
In Liebig's method, both the silver wire and lead chromate need to be placed near the exit of the combustion tube to ensure that they can effectively capture and react with the halogens released during combustion.

**Hint**: Consider where in the apparatus these materials would be most effective in capturing byproducts.

6. **Conclusion**:
Therefore, in Liebig's method for the estimation of carbon and hydrogen when halogens and sulfur are present, both silver wire and lead chromate are kept near the exit of the combustion tube.

**Final Answer**: The correct option is both A and B.

### Summary of the Solution:
In Liebig's method for estimating C and H, when halogens and sulfur are present, both silver wire and lead chromate are kept near the exit of the combustion tube to capture and react with the halogens.