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Question

(i) A black coloured compound (B) is formed on passing hydrogen sulphide through the solution of a compound (A) in ${N H}_{4} O H$ .
(ii) (B) on treatment with hydrochloric acid and potassium chlorate gives (A).
(iii) (A) on treatment with potassium cyanide gives a buff 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.
(v) The solution of (A) is treated with excess of sodium bicarbonate and then bromine water. On cooling and shaking for sometime, a green colour of compound (E) is formed. No change is observed on heating.
Identify (A) &(B) .

{N a O H}_{2} a n d C o S

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{C o C l}_{2} a n d C o C O_{3}

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{M g C l}_{2} a n d N a O H

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{C o C l}_{2} a n d C o S

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{M g C l}_{2} a n d M g N O_{3}

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Solution

The correct option is E ${M g C l}_{2} a n d M g N O_{3}$
(i) CoS (a black coloured compound) is formed on passing hydrogen sulphide through the solution of ${C o C l}_{2}$ in ${N H}_{4} O H$ .
${C o C l}_{2} + H_{2} S + {2 N H}_{4} O H ⟶ C o S b l a c k ↓ + {2 N H}_{4} C l + {2 H}_{2} O$
(ii) $C o S$ on treatment with hydrochloric acid and potassium chlorate gives ${C o C l}_{2}$ .
$C o S + {K C l O}_{3} + 2 H C l ⟶ {C o C l}_{2} + S + H_{2} O$
(iii) ${C o C l}_{2}$ on treatment with potassium cyanide gives a buff coloured precipitate which dissolves in excess of this reagent forming a compound $K_{4} [{C o (C N)}_{6}]$ .
${C o C l}_{2} + 2 K C N ⟶ {C o (C N)}_{2} b u f f c o l o u r e d + 2 K C l$
${C o (C N)}_{2} + 4 K C N ⟶ K_{4} [{C o (C N)}_{6} s o l u b l e]$
(iv) The compound $K_{4} [{C o (C N)}_{6}]$ is changed into a compound $K_{3} [{C o (C N)}_{6}]$ when its aqueous solution is
boiled.
${2 K}_{4} [{C o (C N)}_{6}] + H_{2} O + O ⟶ {2 K}_{3} [{C o (C N)}_{6}] + 2 K O H$
(v) The solution of ${C o C l}_{2}$ is treated with excess of sodium bicarbonate and then bromine water. On cooling and shaking for sometime, a green colour of compound $N a_{3} [{C o ({C O}_{3})}_{3}]$ is formed.
No change is observed on heating.
${2 N a H C O}_{3} ⟶ {N a}_{2} {C O}_{3} + H_{2} O + {C O}_{2} ↑$
${N a}_{2} {C O}_{3} + {C o C l}_{2} ⟶ {C o C O}_{3} + 2 N a C l$
${C o C O}_{3} + {2 N a}_{2} {C O}_{3} ⟶ {N a}_{4} [{C o ({C O}_{3})}_{3}]$
${B r}_{2} + H_{2} O ⟶ 2 H B r + O$
${2 N a}_{4} [{C o ({C O}_{3})}_{3}] + {2 N a H C O}_{3} + O ⟶ {2 N a}_{3} [{C o ({C O}_{3})}_{3} g r e e n, s t a b l e] + {2 N a}_{2} {C O}_{3} + H_{2} O$

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