`H_(2)S` gas when passed through a solution of cations containing `HCl` precipitates the cations of second group in qualitative analysis but not those belonging to the fourth group. It is because

Options

A. Presence of HCl decreases the sulphide ion concentration
B. Sulphides of group IV cations are unstable in HCl
C. Solubility product of group II sulphides is more than that of group IV sulphides
D. Presence of HCl increases the sulphides ion concentration

Answer

To solve the question regarding why `H2S` gas precipitates cations of the second group in qualitative analysis but not those belonging to the fourth group when passed through a solution of cations containing `HCl`, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Groups of Cations**:
- Group 2 cations include: `Hg^2+`, `Cd^2+`, `Bi^3+`, `Cu^2+`, `As^3+`, `Sb^3+`, and `Pb^2+`.
- Group 4 cations include: `Ni^2+`, `Co^2+`, `Mn^2+`, and `Zn^2+`.

2. **Role of `H2S` in Precipitation**:
- `H2S` dissociates in water to produce `H+` and `S^2-` ions. The sulfide ion (`S^2-`) is responsible for precipitating metal cations as their sulfides.

3. **Effect of `HCl` on Sulfide Ion Concentration**:
- When `HCl` is added to the solution, it dissociates to produce `H+` ions. The increase in `H+` ions leads to the common ion effect, which decreases the concentration of sulfide ions (`S^2-`) in the solution.

4. **Solubility Product (Ksp) Considerations**:
- The solubility product constant (Ksp) for the sulfides of Group 2 cations is much lower than that of Group 4 cations. This means that Group 2 sulfides precipitate at lower concentrations of sulfide ions compared to Group 4 sulfides.

5. **Conclusion**:
- Due to the low Ksp of Group 2 sulfides, they precipitate with a relatively low concentration of sulfide ions. The presence of `HCl` reduces the concentration of sulfide ions just enough to precipitate the Group 2 cations while preventing the precipitation of Group 4 cations, which require a higher concentration of sulfide ions to precipitate.

### Final Answer:
`H2S` gas precipitates the cations of the second group in qualitative analysis but not those belonging to the fourth group because the presence of `HCl` decreases the concentration of sulfide ions to a level that is just sufficient to precipitate the Group 2 cations, while the Group 4 cations require a higher concentration of sulfide ions to precipitate.