Question

The concentration of sulphide ion in 0.1M HCl solution saturated with hydrogen sulphide is 1.0 × 10–19 M. If 10 mL of this is added to 5 mL of 0.04 M solution of the following: FeSO4, MnCl2, ZnCl2 and CdCl2. in which of these solutions precipitation will take place?

Answer 1

For precipitation to take place, it is required that the calculated ionic product exceeds the Ksp value. Before mixing: [S2-] = 1.0 × 10-19 M
[M2+] = 0.04M Volume = 10 mL Volume = 5mL
After mixing: [S2-] =?
[M2+] = ?
Volume = (10 + 15) = 15mL Volume = 15mL
[S2-] = $\frac{1.0 × 10 ^{-19} × 10 }{15}$ = 6.67 × 10-20 M
[M2+] = $\frac{0.04 × 5 }{15}$ = 1.33 × 10-2 M
Ionic product = [M2+] [S2-] = (1.33 × 10-2)(6.67 × 10-20) = 8.87 × 10-22
This ionic product exceeds the Ksp of Zns and CdS. Therefore, precipitation will occur in CdCl2 and ZnCl2 solutions.