JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Practical and Analytical Chemistry

Aqua regia (a 3:1 mixture of concentrated $\mathrm{HCl}$ and $\mathrm{HN}{\mathrm{O}}_3$) dissolves metal sulfides that resist ordinary acids, converting the sulfide into the soluble metal chloride: $\mathrm{CoS}\stackrel{\text{aqua regia}}{\to }\mathrm{CoC}{\mathrm{l}}_2$.

Treating the resulting $\mathrm{C}{\mathrm{o}}^{2+}$ solution with $\mathrm{KN}{\mathrm{O}}_2$ in the presence of $\mathrm{N}{\mathrm{H}}_4\mathrm{OH}$ and acetic acid is the classic confirmatory test for cobalt: it oxidizes $\mathrm{C}{\mathrm{o}}^{2+}$ to $\mathrm{C}{\mathrm{o}}^{3+}$ and forms the yellow crystalline precipitate potassium hexanitritocobaltate(III), ${\mathrm{K}}_3[\mathrm{Co}(\mathrm{N}{\mathrm{O}}_2{)}_6]$.

Since this characteristic yellow precipitate forms only from a cobalt salt, compound A must be $\mathrm{CoS}$.