In the wet tests for identification of various cations by precipitation, which transition element cation doesn't belong…

Step 1: Recall the Qualitative Group Divisions

Cations are classified into analytical groups precisely based on their solubility products in varying conditional mediums:

• Group I: $\ce{Ag+}$, $\ce{Pb^{2+}}$, $\ce{Hg2^{2+}}$ (dil. $\ce{HCl}$)
• Group II: $\ce{Cu^{2+}}$, $\ce{Pb^{2+}}$, $\ce{Hg^{2+}}$, $\ce{Bi^{3+}}$, $\ce{Cd^{2+}}$, $\ce{As^{3+}}$, $\ce{Sb^{3+}}$, $\ce{Sn^{2+}}$ ($\ce{H2S}$ in dil. $\ce{HCl}$)
• Group III: $\ce{Fe^{3+}}$, $\ce{Al^{3+}}$, $\ce{Cr^{3+}}$ ($\ce{NH4OH}$ with $\ce{NH4Cl}$)
• Group IV: $\ce{Zn^{2+}}$, $\ce{Mn^{2+}}$, $\ce{Ni^{2+}}$, $\ce{Co^{2+}}$ ($\ce{H2S}$ in $\ce{NH4OH}$)

Step 2: Match options with their groups

• $\ce{Fe^{3+}}$: Precipitates as its hydroxide, $\ce{Fe(OH)3}$, belonging rigorously to Group III.
• $\ce{Zn^{2+}}$: Precipitates as $\ce{ZnS}$ belonging to Group IV.
• $\ce{Co^{2+}}$: Precipitates as $\ce{CoS}$ belonging to Group IV.
• $\ce{Ni^{2+}}$: Precipitates as $\ce{NiS}$ belonging to Group IV.

Step 3: Conclusion

The cation uniquely outside Group IV is $\ce{Fe^{3+}}$.

Key Points to Remember:

• Elements precipitating directly as basic hydroxides with limited $\ce{OH-}$ ions form Group III (Fe, Al, Cr).
• Elements precipitating strictly as metal sulphides in basic conditions securely define Group IV (Zn, Mn, Ni, Co).