Question

(i) A white precipitate $(B)$ is formed when a mineral $(A)$ is boiled with ${Na}_{2} {CO}_{3}$ solution.

(ii) The precipitate is filtered and the filtrate contains two compounds $(C)$ and $(D)$ . The compound $(C)$ is removed by crystallisation and when ${CO}_{2}$ is passed through the mother liquor left $(D)$ changes to $(C)$ .

(iii) The compound $(C)$ on strong heating gives two compounds $(D)$ and $(E)$

(iv) $(E)$ on heating with cobalt oxide produces blue coloured substance $(F)$ Identify $(A)$ to $(F)$ and give chemical equations for the reactions at steps (i) to (iv).

Accepted Answer

The mineral $(A)$ and other compounds are:

(A) ${Ca}_{2} B_{6} O_{11} \cdot 5 H_{2} O$ (Colemanite)

(B) ${CaCO}_{3}$ (Calcium carbonate)

(C) ${Na}_{2} B_{4} O_{7}$ (Borax)

(D) ${NaBO}_{2}$ (Sodium metaborate)

(E) $B_{2} O_{3}$ (Boric anhydride)

(F) $Co {({BO}_{2})}_{2}$ (Cobalt metaborate)

The reactions are.

${Ca}_{2} B_{6} O_{11} + 2 {Na}_{2} {CO}_{3} (aq .) ⟶ 2 Ca \underset{(B)}{{CO}_{3}} ↓ +$ $\underset{Soluble}{\underset{⏟}{\underset{(C)}{{Na}_{2} B_{4} O_{7}} + \underset{(D)}{{NaBO}_{2}}}}$

$\underset{(D)}{4 {NaBO}_{2}} + {CO}_{2} ⟶ \underset{(C)}{{Na}_{2} B_{4} O_{7}} + {Na}_{2} {CO}_{3}$

$\underset{(C)}{{Na}_{2} B_{4} O_{7}} \overset{Δ}{⟶} \underset{(D)}{2 {NaBO}_{2}} + \underset{(E)}{B_{2} O_{3}}$

$\underset{(E)}{B_{2} O_{3}} + CoO ⟶ \underset{(Bule) (a black mineral)}{Co {({BO}_{2})}_{2}}$

Important Questions on The p-Block Elements

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