An aqueous blue coloured solution of a transition metal sulphate reacts with ${\mathrm{H}}_2\mathrm{S}$ in acidic medium to give a black precipitate $\mathrm{A}$, which is insoluble in the warm aqueous solution of $\mathrm{KOH}$. The blue solution, on treatment with $\mathrm{KI}$ in weakly acidic medium, turns yellow and produces a white precipitate $\mathrm{B}$. Identify the transition metal ion. Write the chemical reactions involved in the formation of $\mathrm{A}$ and $\mathrm{B}$.

What will be the hybridization in compound $\left(\mathrm{A}\right)$ (this question is from the book does not give details and is incorrect. Hence, it has been reframed along with options). An unknown compound (A) when treated with concentrated sulphuric acid, gives light brown fumes which intensify on the addition of copper turnings. When an aqueous solution of the compound is treated with freshly prepared ${\mathrm{FeSO}}_4$, followed by concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$, it yields a brown complex at the junction of the two liquids. When the compound is heated with sodium hydroxide solution, it evolves a colourless pungent gas. In another test tube, the solution of the compound is mixed with Nessler's reagent to form a brown precipitate. What is the hybridization of the compound (A)?