p-Block Elements-I 1 Question 13

13. The crystalline form of borax has

(2016 Adv.)

(a) tetranuclear $\left[\mathrm{B} _{4} \mathrm{O} _{5}(\mathrm{OH}) _{4}\right]^{2-}$ unit

(b) all boron atoms in the same plane

(c) equal number of $s p^{2}$ and $s p^{3}$ hybridised boron atoms

(d) one terminal hydroxide per boron atom

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Answer:

Correct Answer: 13. (b)

Solution:

  1. $\mathrm{Na} _{2} \mathrm{B} _{4} \mathrm{O} _{7} \cdot 10 \mathrm{H} _{2} \mathrm{O}$ (borax) is actually made of two tetrahedral and two triangular units, and is actually written as $\mathrm{Na} _{2}\left[\mathrm{B} _{4} \mathrm{O} _{5}(\mathrm{OH}) _{4}\right] \cdot 5 \mathrm{H} _{2} \mathrm{O}$.

(a) Thus, correct.

(b) Boron atoms are in different planes thus, incorrect.

(c) Two $s p^{2}$ and two $s p^{3}$-hybridised B atoms thus, correct.

(d) Each boron has one - $\mathrm{OH}$ group thus, correct.



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