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. (a,c,d)
Solution:
- $\mathrm{Na} _{2} \mathrm{B} _{4} \mathrm{O} _{7} \cdot 10 \mathrm{H} _{2} \mathrm{O}$ (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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