Cations are classified into varius group on the basis of their behaviour against some reagents .The group reagent used for the classifaction of most common cation are `HCI,H_(2)S,NH_(4)OH,(NH_(4))_(2)CO_(3)` Classification is based on whether a cation reacts with these reagents by the formation of precipitates or not .
An aqueous solution contain `Hg^(2+),Hg_(2)^(2+),Pb^(2+)`.THe addition of `2M HCI` will procipitate.

Options

A. `HgCI_(2)`only
B. `PbCI_(2)"only"^(-)`
C. `PbCI_(2)` and `Hg_(3)CI_(2)`
D. `PbCI_(2)` and `CdCI_(2)`

Explanation

To solve the question regarding the precipitation of cations in an aqueous solution containing \( \text{Hg}^{2+} \), \( \text{Hg}_2^{2+} \), and \( \text{Pb}^{2+} \) upon the addition of \( 2M \, \text{HCl} \), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Cations Present**:
The cations in the solution are \( \text{Hg}^{2+} \) (mercury(II)), \( \text{Hg}_2^{2+} \) (mercurous), and \( \text{Pb}^{2+} \) (lead(II)).

2. **Understand the Role of HCl**:
Hydrochloric acid (HCl) is a strong acid that provides chloride ions (\( \text{Cl}^- \)). It acts as a group reagent for the first group of cations in qualitative analysis.

3. **Determine the Precipitation Reaction**:
When \( \text{HCl} \) is added to the solution, it reacts with the cations to form their respective chlorides. The relevant reactions are:
- For \( \text{Hg}^{2+} \):
\[ \text{Hg}^{2+} + 2\text{Cl}^- \rightarrow \text{Hg}_2\text{Cl}_2 \, (s) \]
(Mercurous chloride, which is a white precipitate)
- For \( \text{Hg}_2^{2+} \):
\[ \text{Hg}_2^{2+} + 2\text{Cl}^- \rightarrow \text{Hg}_2\text{Cl}_2 \, (s) \]
(Mercurous chloride, which is also a white precipitate)
- For \( \text{Pb}^{2+} \):
\[ \text{Pb}^{2+} + 2\text{Cl}^- \rightarrow \text{PbCl}_2 \, (s) \]
(Lead(II) chloride, which is a white precipitate)

4. **Identify the Precipitates Formed**:
Upon the addition of \( 2M \, \text{HCl} \), both \( \text{Hg}_2\text{Cl}_2 \) (mercurous chloride) and \( \text{PbCl}_2 \) (lead(II) chloride) will precipitate out of the solution.

5. **Conclusion**:
The precipitates formed when \( 2M \, \text{HCl} \) is added to the solution containing \( \text{Hg}^{2+} \), \( \text{Hg}_2^{2+} \), and \( \text{Pb}^{2+} \) are \( \text{Hg}_2\text{Cl}_2 \) and \( \text{PbCl}_2 \). Thus, the answer is that both \( \text{Hg}_2\text{Cl}_2 \) and \( \text{PbCl}_2 \) will precipitate.

### Final Answer:
The addition of \( 2M \, \text{HCl} \) will precipitate \( \text{Hg}_2\text{Cl}_2 \) (mercurous chloride) and \( \text{PbCl}_2 \) (lead(II) chloride).

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