Find the compound 'A' from the following reaction sequences. {A} {{aqua-regia}} {B} [{(2) AcOH}]{{(1) } {KNO2},…

Step 1: Understand the Problem

The given reaction sequence is a characteristic qualitative analysis test for a specific metal sulfide (Compound A), particularly one involving a yellow precipitate upon treatment with $\ce{KNO2}$ and acetic acid.

Step 2: Identify the characteristic test (Yellow PPT with $\ce{KNO2}$)

The formation of a yellow precipitate with potassium nitrite ($\ce{KNO2}$) in the presence of acetic acid (AcOH) is the classic confirmatory test for Cobalt ($\ce{Co^{2+}}$). The yellow precipitate is potassium hexanitritocobaltate(III), also known as Fischer's salt.

Step 3: Correlate with the reaction sequence

1. Compound A is dissolved in aqua-regia. Cobalt sulfide ($\ce{CoS}$) and Nickel sulfide ($\ce{NiS}$) belong to Group IV, but they do not dissolve readily in dilute valid acids like $\ce{HCl}$, hence they are dissolved using a strong oxidizing mixture like aqua-regia to form respective soluble chlorides (Compound B = $\ce{CoCl2}$). $\ce{3CoS + 2HNO3 + 6HCl -> 3CoCl2 + 2NO + 3S + 4H2O}$

2. The cobalt chloride solution is treated with ammonium hydroxide to neutralize excess acid, and then reacted with potassium nitrite ($\ce{KNO2}$) and acetic acid.

   $\ce{Co^{2+} + 7NO2- + 3K+ + 2H+ -> K3[Co(NO2)6] v + NO + H2O}$

   This gives the yellow precipitate of $\ce{K3[Co(NO2)6]}$. Nickel does not give this test.

Step 4: Conclusion

The compound A must be $\ce{CoS}$.

Key Points to Remember:

• $\ce{CoS}$ and $\ce{NiS}$ are black precipitates that require aqua regia to dissolve.
• Cobalt gives a characteristic yellow precipitate of Fischer's salt ($\ce{K3[Co(NO2)6]}$) with $\ce{KNO2}$ and acetic acid.
• Nickel forms a brilliant red precipitate with Dimethylglyoxime (DMG) instead.