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 with NH4OH. To a neutral solution of Co2+, acetic acid and saturated solution of KNO2 are added which results in the formation of yellow precipitate of K3[Co(NO2)6] Co2++7NO2−+2H++3 K+→K3[Co(NO2)6]+NO↑+H2O