(i)A black coloured compound `(B)` is formed on passing `H_(2)S` through the solution of a compound `(A)` in `NH_(4)OH` (ii)`(B)` on treatment with `H

Monjur Alahi

(i)A black coloured compound `(B)` is formed on passing `H_(2)S` through the solution of a compound `(A)` in `NH_(4)OH`
(ii)`(B)` on treatment with `HCl` and potassium chlorate or aquaregia gives `(A)`
(iii)`(A)` on treatment with `KCN` gives a buff/reddish-brown coloured precipitate which dissolves in excess of this reagent forming a compound `( C)`.
(iv)The compound `( C)` is changed into a compound `(D)` when its aqueous solution is boiled in air.
(v)The solution of `(A)` was treated with excess of `NaHCO_(3)` & then with bromine water. On cooling & shaking for some time, a green colour of compound `(E)` is formed.No change is observed on heating.
Identify `(A)` to `(E)` and give chemical equations.


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Correct Answer - (A)=`CoCl_(2)` , (B)=`CoS` , (C )=`K_(4)[Co(CN)_(6)]` , (D)=`K_(3)[Co(CN)_(6)]`, (E)=`Na_(3)[Co(CO_(3))_(3)]`
(i)`Co_(2+) + S^(2-) overset (NH_(4)OH//NH_(4)Cl) to CoS darr` (black)
(ii)`CoS darr + HNO_(3)+ 3HCl to Co^(2+) +S darr + NOCl uarr + 2Cl^(-) + 2H_(2)O`
(iii)`Co^(2+) + 2CN^(-) to Co(CN)_(3) darr` (reddish-brown)
`Co(CN)_(2) darr + 4CN^(-) to 4[Co(CN)_(6)]^(4-)`(brown solution)
(iv)`4[Co(CN)_(6)]^(4-) + H_(2)O + O_(2) to 4[Co(CN)_(6)]^(3-)` (yellow solution)+`4OH^(-)`
(v)`Co^(2+) + 6HCO_(3)^(-) to [Co(CO_(3))_(3)]^(4-) + 3H_(2)O + 3CO_(2)`
`[Co(CO_(3))_(3)]^(4-) overset([O])to[Co(CO_(3))_(3)]^(3-)` (green solution)

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