Compound (A) in the given reaction sequence is likely to be CoS. CoS+HNO3+3HCl⟶Co2+‌+S↓+NOCl↑‌+2Cl−+2H2O The above solution is neutralised withNH4OH. To a neutral solution ofCo2+, acetic acid and saturated solution ofKNO2are added which results in the formation of yellow precipitate ofK3[Co(NO2)6] Co2++7NO2−+2H++3K+⟶K3[Co(NO2)6]+NO↑+H2O