An aqueous solution contains `Ni^(2+), Co^(2+), Pb^(2+)` ions at equal concentrations. The solubility product of NiS, PbS and CoS in water at `25^@C` are respectively given below. Indicate which of these ions will be precipitated first and last when sulphide concentration is progressively increased from zero ?`[K_(sp) of NiS=3xx10^(-19), K_(sp) of CoS=4xx10^(-21), K_(sp) of PbS=3xx10^(-28)`

Options

A. NiS and PbS
B. NiS and CoS
C. CoS and NiS
D. PbS and NiS

Explanation

To determine which ions will precipitate first and last when the concentration of sulfide ions (S²⁻) is progressively increased in a solution containing equal concentrations of Ni²⁺, Co²⁺, and Pb²⁺ ions, we will compare the solubility products (Ksp) of their respective sulfides: NiS, CoS, and PbS.

### Step-by-Step Solution:

1. **Identify the Ksp Values:**
- Ksp of NiS = 3 × 10^(-19)
- Ksp of CoS = 4 × 10^(-21)
- Ksp of PbS = 3 × 10^(-28)

2. **Understand the Precipitation Process:**
- Precipitation occurs when the product of the ion concentrations exceeds the Ksp of the salt.
- The lower the Ksp value, the less soluble the salt is, meaning it will precipitate first when sulfide ions are added.

3. **Compare Ksp Values:**
- Ksp of PbS (3 × 10^(-28)) is the lowest, indicating that PbS will precipitate first.
- Ksp of CoS (4 × 10^(-21)) is higher than that of PbS but lower than that of NiS, indicating that CoS will precipitate next.
- Ksp of NiS (3 × 10^(-19)) is the highest, indicating that NiS will precipitate last.

4. **Conclusion:**
- The order of precipitation when sulfide concentration is increased is:
- **First:** PbS (Lead Sulfide)
- **Second:** CoS (Cobalt Sulfide)
- **Last:** NiS (Nickel Sulfide)

### Final Answer:
- **First to precipitate:** Lead Sulfide (PbS)
- **Last to precipitate:** Nickel Sulfide (NiS)