p-Block elementsHardBloom L1

Question

Ca2B6O11 + Na2CO3 $\overset{\quad\Delta\quad}{\rightarrow}$ [X] + CaCO3 + NaBO2 (Unbalanced equation)

Correct statement for [X]

Options

A.Structure of anion of crystalline (X) has one boron atom sp3 hybridised and other three boron atoms sp2 hybridised
B.(X) with NaOH(aq.) gives a compound which on reaction with H2O2 in alkaline medium yields a compound used as brightner in soaps
C.Hydrolysis of (X) with HCl or H2SO4 yields a compound which on reaction with HF gives fluoroboric acid
D.[X] on heating with cobalt salt in oxidising flame gives blue coloured bead

Solution

Sol. The compound X is borax (Na2​B4​O7​). It is prepared by boiling finely powdered colemanite mineral (Ca2​B6​O11​) with sodium carbonate solution.

Ca2​B6​O11​+2Na2​CO3​→2CaCO3​+Na2​B4​O7​+2NaBO2​

The statements (B), (C) and (D) are correct. The statement (A) is incorrect.

(A) The structure of anion of crystalline [X] has two boron atoms sp3 hybridized and other two boron atoms sp2 hybridized.

(B) X with NaOH(aq), gives sodium metaborate NaBO2​ which on reaction with hydrogen peroxide in alkaline medium yields hexahydrate sodium tetraaquadiperoxoborate (III).

2NaBO2​ + 2H2​O2 ​ + 6H2​O → Na2​[B2​(O2​)2​(OH)4​]•6H2​O

(C) Hydrolysis of [X] with HCl or H2​SO4​ yields boric acid which on reaction with HF gives fluoroboric acid.

Na2​B4​O7 ​+ H2​SO4 ​→ Na2​SO4​ + H2​B4​O7​

H2​B4​O7​ + 5H2​O → 4H3​BO3​

H3​BO3​ + 4HF → H[BF4​] + 3H2​O

(D) [X] on heating with chromium slats in oxidizing flame gives green colored bead.

(Cr2​O)​3+3B2​O3​→2Cr(BO2​)3​(chromium metaborate green)

This is borax bead test.

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