During the qualitative analysis of salt with cation (Y2+), addition of a reagent (X) to alkaline solution of the salt…

Step 1: Analyze the observations The problem states that an unknown cation $\ce{Y^{2+}}$ gives a bright red precipitate when reagent $\text{X}$ is added in an alkaline medium.

Step 2: Evaluate the provided options

1. Dimethylglyoxime (DMG) and $\ce{Ni^{2+}}$: Nickel(II) ions exclusively react with DMG in an alkaline (ammoniacal) medium to form a remarkably distinctive bright/rosy red precipitate of Nickel dimethylglyoximate ($\ce{[Ni(DMG)2]}$).
2. Dimethylglyoxime and $\ce{Co^{2+}}$: Cobalt(II) forms a soluble brown complex with DMG, not a bright red precipitate.
3. Nessler's reagent and $\ce{Hg^{2+}}$: Nessler's reagent itself contains mercury ($\ce{K2HgI4}$), but it is used to detect ammonia ($\ce{NH3}$) or ammonium ($\ce{NH4+}$), producing a brown precipitate, not a red one.
4. Nessler's reagent and $\ce{Ni^{2+}}$: No significant red precipitating reaction exists between Nessler's reagent and $\ce{Ni^{2+}}$.

Step 3: Conclusion The reagent X is undeniably Dimethylglyoxime, and the specific cation is $\ce{Ni^{2+}}$.

Key Points to Remember:

• Nickel(II) + DMG in $\ce{NH4OH}$ $\rightarrow$ Rosy/Bright red precipitate.
• $\ce{[Ni(DMG)2]}$ is a square planar, diamagnetic complex heavily stabilized by intramolecular hydrogen bonds.