Question

In the wet tests for identification of various cations by precipitation, which transition element cation doesn’t belong to group IV in qualitative inorganic analysis?

• A. \(Fe^{3+}\)
• B. \(Zn^{2+}\)
• C. \(Co^{2+}\)
• D. \(Ni^{2+}\)

Answer 1

The Correct Option is A

Step 1: Understanding the Grouping of Cations in Qualitative Analysis
In qualitative inorganic analysis, cations are classified into groups based on their precipitation behavior with specific reagents. The classification is as follows:
Group III: Includes \(\text{Fe}^{3+}\), \(\text{Al}^{3+}\), and \(\text{Cr}^{3+}\), which precipitate as hydroxides in the presence of \(\text{NH}_4\text{OH}\) and \(\text{NH}_4\text{Cl}\).
Group IV: Includes \(\text{Zn}^{2+}\), \(\text{Co}^{2+}\), and \(\text{Ni}^{2+}\), which precipitate as sulfides in the presence of \(\text{H}_2\text{S}\) in neutral or slightly acidic medium.
Step 2: Analysis of the Cations
\(\text{Fe}^{3+}\): Belongs to Group III, as it forms \(\text{Fe(OH)}_3\) precipitate with \(\text{NH}_4\text{OH}\).
\(\text{Zn}^{2+}\), \(\text{Co}^{2+}\), \(\text{Ni}^{2+}\): Belong to Group IV, as they precipitate as sulfides (\(\text{ZnS}\), \(\text{CoS}\), \(\text{NiS}\)) with \(\text{H}_2\text{S}\).
Conclusion: \(\text{Fe}^{3+}\) does not belong to Group IV. Therefore, the correct answer is (1) \(\text{Fe}^{3+}\).