Why only `As^(+3)` gets precipitated as `As_(2) S_(3)` and not `Zn^(2+)` as ZnS when `H_(2)`S is passed through an acidic solution containing `As^(+3) and Zn^(+2)`?

Options

A. Solubility product of `As_(2) S_(3)` is less than that of ZnS
B. Enough `As^(+3)` are present in acidie medium
C. Zinc salt does not ionise in acidic medium
D. Solubility product changes in presence of an acid.

Answer

To understand why only \( \text{As}^{3+} \) gets precipitated as \( \text{As}_2\text{S}_3 \) and not \( \text{Zn}^{2+} \) as \( \text{ZnS} \) when \( \text{H}_2\text{S} \) is passed through an acidic solution containing both ions, we need to analyze the solubility products (Ksp) of the respective sulfides.

### Step-by-Step Solution:

1. **Identify the Ions in Solution**:
The solution contains \( \text{As}^{3+} \) and \( \text{Zn}^{2+} \) ions.

2. **Understanding the Precipitation Reaction**:
When \( \text{H}_2\text{S} \) is introduced into the solution, it can react with metal ions to form their respective sulfides:
- \( \text{As}^{3+} + \text{H}_2\text{S} \rightarrow \text{As}_2\text{S}_3 \) (precipitate)
- \( \text{Zn}^{2+} + \text{H}_2\text{S} \rightarrow \text{ZnS} \) (potential precipitate)

3. **Solubility Product (Ksp) Concept**:
The precipitation of a salt occurs when the ionic product of the ions exceeds the solubility product (Ksp) of that salt.
- For \( \text{As}_2\text{S}_3 \), the Ksp is relatively low.
- For \( \text{ZnS} \), the Ksp is higher compared to that of \( \text{As}_2\text{S}_3 \).

4. **Comparison of Ksp Values**:
- The Ksp of \( \text{As}_2\text{S}_3 \) is less than that of \( \text{ZnS} \). This means that \( \text{As}_2\text{S}_3 \) is less soluble in water compared to \( \text{ZnS} \).

5. **Ionic Product Calculation**:
When \( \text{H}_2\text{S} \) is added, the ionic product for \( \text{As}^{3+} \) and \( \text{S}^{2-} \) ions will exceed the Ksp of \( \text{As}_2\text{S}_3 \) first, leading to its precipitation.
- The ionic product for \( \text{Zn}^{2+} \) and \( \text{S}^{2-} \) ions does not exceed the Ksp of \( \text{ZnS} \) under the same conditions.

6. **Conclusion**:
Therefore, \( \text{As}^{3+} \) precipitates as \( \text{As}_2\text{S}_3 \) because its Ksp is lower than that of \( \text{ZnS} \), which remains soluble in the solution.

### Final Answer:
Only \( \text{As}^{3+} \) gets precipitated as \( \text{As}_2\text{S}_3 \) because the Ksp of \( \text{As}_2\text{S}_3 \) is lower than that of \( \text{ZnS} \), allowing \( \text{As}^{3+} \) to exceed its Ksp and precipitate, while \( \text{Zn}^{2+} \) remains in solution.