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.23 The concentration of S2−, at which FeS begins to precipitate from the mixture
having 0.1 M Fe2+ is
Options:
(A) 1.0 × 10−17 M
(B) 1.0 × 10−18 M
(C) 1.0 × 10−19 M
(D) 1.0 × 10−20 M
Concentration of S2− Required for FeS Precipitation
In inorganic chemistry, precipitation of metal sulphides depends on
the solubility product (Ksp).
Precipitation begins when the ionic product becomes equal to
Ksp. This problem determines the sulphide ion concentration
required to start the precipitation of FeS.
Given Data
- Ksp (FeS) = 1.0 × 10−19
- Ksp (ZnS) = 4.5 × 10−24
- Ksp (CuS) = 4.0 × 10−38
- Ksp (HgS) = 3.0 × 10−53
- [Fe2+] = 0.1 M
Key Concept
For the salt FeS:
FeS (s) ⇌ Fe2+ + S2−
Ksp = [Fe2+][S2−]
Precipitation starts when the ionic product equals Ksp.
Step-by-Step Calculation
Ksp (FeS) = 1.0 × 10−19
[Fe2+] = 0.1 M = 1.0 × 10−1
[S2−] = Ksp / [Fe2+]
[S2−] = (1.0 × 10−19) / (1.0 × 10−1)
[S2−] = 1.0 × 10−18 M
Correct Answer
Option (B): 1.0 × 10−18 M
Summary
| Parameter | Value |
|---|---|
| Ksp of FeS | 1.0 × 10−19 |
| [Fe2+] | 0.1 M |
| Required [S2−] | 1.0 × 10−18 M |
| Correct Option | (B) |
Conclusion
FeS begins to precipitate when the ionic product equals its solubility
product. For a solution containing 0.1 M Fe2+, the required
sulphide ion concentration is:
1.0 × 10−18 M
Hence, the correct answer is Option (B).
