Find the compound ' A ' from the following reaction sequences. 

yellow ppt

The sulphides of the basic-radical cations of the IV group of qualitative analysis precipitate when $$H_2S$$ is passed through the alkaline (NH4Cl + NH4OH) medium.

Colour of fresh sulphide precipitates
  • $$\text{Zn}^{2+} \rightarrow \text{ZnS}$$ : white
  • $$\text{Mn}^{2+} \rightarrow \text{MnS}$$ : flesh-coloured / light pink
  • $$\text{Ni}^{2+} \rightarrow \text{NiS}$$ : black
  • $$\text{Co}^{2+} \rightarrow \text{CoS}$$ : black

The problem statement says that in the subsequent steps a yellow precipitate is obtained. This yellow solid is not the sulphide itself, but the confirmatory test product for cobalt(II):

$$\text{Co}^{2+} + 6\,\text{NO}_2^- + 3\,\text{K}^+ + \text{O} \;\longrightarrow\; \text{K}_3\left[\text{Co}\left(\text{NO}_2\right)_6\right] \downarrow$$

Potassium hexanitritocobaltate(III), $$\text{K}_3[\text{Co}(\text{NO}_2)_6]$$, is a canary-yellow crystalline precipitate and is the standard confirmatory test for $$\text{Co}^{2+}$$ ions.

Therefore the unknown sulphide that finally gives this yellow precipitate must contain cobalt. Among the given options, the only cobalt sulphide is $$\textbf{CoS}$$.

Hence, compound ‘A’ is $$\boxed{\text{CoS}}$$.

Option A which is: CoS