The pair(s) of ions where BOTH the ions are precipitated upon passing H2S gas in presence of dilute HCl, is(are)
Cu2+, Pb2+, Hg2+, Bi3+ give ppt with H2S in presence of dilute HCl.
This question tests your understanding of qualitative inorganic analysis, specifically the precipitation of sulfides in acidic medium (using H2S in presence of dilute HCl).
Key Concept: Hydrogen sulfide (H2S) is used as a group reagent to precipitate certain metal ions as their sulfides. The solubility of these sulfides depends on the pH. In the presence of dilute HCl (an acidic medium), the concentration of S2- ions is suppressed due to the common ion effect (H+ ions from HCl suppress the dissociation of H2S). Only the sulfides of metals with very low solubility product (Ksp) precipitate in this acidic medium. These belong to Group II in the qualitative analysis scheme.
The common Group II cations that precipitate as sulfides in acidic medium are: Hg2+, Cu2+, Bi3+, Cd2+, As3+, Sb3+, Sn2+, and Pb2+.
Let's analyze each pair:
Step 1: Analyze Pair 1 - Cu2+, Pb2+
Both Cu2+ and Pb2+ are Group II cations. Cu2+ precipitates as black CuS. Pb2+ precipitates as black PbS. Conclusion: Both ions are precipitated.
Step 2: Analyze Pair 2 - Ba2+, Zn2+
Ba2+ is a Group V cation and does not form a precipitate with H2S under any condition. Zn2+ is a Group IV cation; its sulfide (ZnS) has a higher Ksp and only precipitates in basic medium, not in acidic medium. Conclusion: Neither ion is precipitated in acidic medium.
Step 3: Analyze Pair 3 - Bi3+, Fe3+
Bi3+ is a Group II cation and precipitates as black Bi2S3. Fe3+ is a Group III cation; it is precipitated as the hydroxide (Fe(OH)3), not the sulfide, in the basic medium of the next group. Fe2S3 is unstable and hydrolyzes in water. Conclusion: Only Bi3+ is precipitated. Fe3+ is not.
Step 4: Analyze Pair 4 - Hg2+, Bi3+
Hg2+ is a Group II cation and precipitates as black HgS. Bi3+ is also a Group II cation and precipitates as black Bi2S3. Conclusion: Both ions are precipitated.
Final Answer: The correct pairs where both ions are precipitated are the first pair (Cu2+, Pb2+) and the fourth pair (Hg2+, Bi3+).
Qualitative Analysis: This is a classical scheme for identifying cations present in a mixture. Cations are separated into groups based on the solubility of their chlorides, sulfides, hydroxides, and carbonates.
Group II Cations: Precipitated as sulfides from a solution acidified with dilute HCl. The precipitation occurs due to the very low solubility product of their sulfides, represented by the general reaction:
Solubility Product (Ksp): The product of the molar concentrations of the constituent ions, each raised to the power of its stoichiometric coefficient in the equilibrium equation. For a sulfide MS, Ksp = [M2+][S2-]. A very low Ksp value is necessary for precipitation in acidic medium.
For the given aqueous reactions, which of the statement(s) is (are) true?
An aqueous solution of metal ion M1 reacts separately with reagents Q and R in excess to give tetrahedral and square planar complexes, respectively. An aqueous solution of another metal ion M2 always forms tetrahedral complexes with these reagents. Aqueous solution of M2 on reaction with reagent S gives white precipitate which dissolves in excess of S. The reactions are summarized in the scheme given below :
In the following reaction sequence in aqueous solution, the species X, Y and Z, respectively, are
