ZnS is not precipitated by passing `H_(2)S` in acidic medium but CuS is precipitated . The reason for it is

Options

A. `K_(sp) CuS lt lt K_(sp) ZnS`
B. `K_(sp)CuS gt gt K_(sp) ZnS`
C. `K_(p) CuS =K_(sp) ZnS`
D. None of these

Explanation

To understand why ZnS is not precipitated by passing H₂S in acidic medium while CuS is precipitated, we need to analyze the solubility product constant (Ksp) for both compounds and the conditions under which they precipitate.

### Step-by-Step Solution:

1. **Understanding Ksp**:
- The solubility product constant (Ksp) is a measure of the solubility of a sparingly soluble salt. It is defined as the product of the molar concentrations of the ions in a saturated solution, each raised to the power of their coefficients in the balanced equation.
- For ZnS:
\[ K_{sp} = [Zn^{2+}][S^{2-}] \]
- For CuS:
\[ K_{sp} = [Cu^{2+}][S^{2-}] \]

2. **Comparing Ksp Values**:
- The Ksp value for CuS is lower than that for ZnS. This means that CuS is less soluble in water compared to ZnS.
- As a result, when H₂S is introduced into a solution containing Cu²⁺ ions, the ionic product (IP) of CuS exceeds its Ksp, leading to the precipitation of CuS.

3. **Effect of Acidic Medium**:
- In an acidic medium, the concentration of H⁺ ions increases. This causes the equilibrium of the sulfide ions (S²⁻) to shift, as H⁺ ions react with S²⁻ to form H₂S.
- The presence of H⁺ ions reduces the concentration of S²⁻ ions, which is essential for the precipitation of both ZnS and CuS.

4. **Precipitation of CuS**:
- Since CuS has a lower Ksp, even a small concentration of S²⁻ ions can lead to the precipitation of CuS when H₂S is passed through the solution.
- In contrast, the Ksp of ZnS is higher, meaning that a higher concentration of S²⁻ ions is required to initiate precipitation. In acidic conditions, the concentration of S²⁻ is too low to exceed the Ksp of ZnS, preventing its precipitation.

5. **Conclusion**:
- Therefore, when H₂S is passed in an acidic medium, CuS precipitates due to its lower Ksp, while ZnS does not precipitate because the conditions do not allow for sufficient S²⁻ concentration to exceed its Ksp.