Chemistry - Complete d-block Question with Solution | TestHub

Compound $(A)$ in the given reaction sequence is likely to be CoS.
$\begin{array}{rl}\mathrm{CoS}+{\mathrm{HNO}}_3+3\mathrm{HCl}\to {\mathrm{Co}}^{2+}& +\mathrm{S}\downarrow +\mathrm{NOCl}\uparrow \\ & +2{\mathrm{Cl}}^{-}+2{\mathrm{H}}_2\mathrm{O}\end{array}$
The above solution is neutralised with ${\mathrm{NH}}_4\mathrm{OH}$.
To a neutral solution of ${\mathrm{Co}}^{2+}$, acetic acid and saturated solution of ${\mathrm{KNO}}_2$ are added which results in the formation of yellow precipitate of ${\mathrm{K}}_3\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]$
${\mathrm{Co}}^{2+}+7{\mathrm{NO}}_2^{-}+2{\mathrm{H}}^{+}+3{\mathrm{K}}^{+}\to {\mathrm{K}}_3\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]+\mathrm{NO}\uparrow +{\mathrm{H}}_2\mathrm{O}$