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. $Z n^{2+}$
• B. $Fe ^{3+}$
• C. $Ni ^{2+}$
• D. $Co ^{2+}$

Answer 1

The Correct Option is B

In qualitative inorganic analysis, cations are classified into different groups based on their chemical properties, particularly their reactions with certain reagents. In the context of wet tests for cations, Group IV cations typically include those that form precipitates with hydrogen sulfide (\(H_2S\)) in the presence of ammonium hydroxide (\(NH_4OH\) at neutral or slightly alkaline pH.

Let's examine each of the provided cations:

1. \(Zn^{2+}\): Zinc ions typically form a white precipitate of zinc sulfide (\(ZnS\)) in the presence of \(H_2S\). Zinc, although sometimes confused with Group II cations, generally appears in Group IV in certain classification schemes.
2. \(Fe^{3+}\): Iron(III) ions do not belong to Group IV because they do not precipitate with \(H_2S\) at neutral pH. Instead, they commonly form rust-colored precipitates such as ferric hydroxide (\(Fe(OH)_3\)) when treated with a base. Iron(III) ions are typically found in Group III.
3. \(Ni^{2+}\): Nickel ions form a black precipitate of nickel sulfide (\(NiS\)) with \(H_2S\), thus commonly categorized in Group IV.
4. \(Co^{2+}\): Cobalt ions also form a black precipitate, cobalt sulfide (\(CoS\)), with \(H_2S\) and are classified in Group IV.

Based on the analysis, \(Fe^{3+}\) does not belong to Group IV, as it does not precipitate with \(H_2S\) under those conditions. Instead, it belongs to Group III due to its reactions in qualitative analysis.

Hence, the correct answer is \(Fe^{3+}\).