The solubility products of FeS, ZnS, CuS and HgS are
1.0 × 10−19, 4.5 × 10−24, 4.0 × 10−38 and 3.0 × 10−53
respectively.
Q.22 H2S is passed through an aqueous solution containing all the four metal ions.
The metal ion that precipitates first is
Options:
(A) Fe2+
(B) Zn2+
(C) Cu2+
(D) Hg2+
Which Metal Ion Precipitates First When H2S Is Passed?
In qualitative inorganic analysis, selective precipitation of metal
sulphides using H2S depends on the
solubility product (Ksp).
The sulphide with the lowest Ksp precipitates first.
Given Solubility Product Values
- FeS = 1.0 × 10−19
- ZnS = 4.5 × 10−24
- CuS = 4.0 × 10−38
- HgS = 3.0 × 10−53
Key Concept: Solubility Product (Ksp)
Lower the value of Ksp, lower is the solubility of the compound.
Hence, the compound with the lowest Ksp precipitates first.
Comparison of Ksp Values
HgS < CuS < ZnS < FeS
3.0 × 10−53 < 4.0 × 10−38 <
4.5 × 10−24 < 1.0 × 10−19
Therefore, HgS has the lowest solubility.
Correct Answer
Option (D): Hg2+
Order of Precipitation
| Metal Ion | Sulphide | Ksp | Precipitation Order |
|---|---|---|---|
| Hg2+ | HgS | 3.0 × 10−53 | 1st |
| Cu2+ | CuS | 4.0 × 10−38 | 2nd |
| Zn2+ | ZnS | 4.5 × 10−24 | 3rd |
| Fe2+ | FeS | 1.0 × 10−19 | Last |
Conclusion
When H2S is passed through a solution containing multiple
metal ions, the sulphide with the lowest Ksp forms first.
Since HgS has the smallest solubility product, Hg2+
precipitates first.
Hence, the correct answer is Option (D).
