p-Block Elements-I 1 Question 14
14. The correct statement(s) for orthoboric acid is/are(2014 Adv.)
(a) It behaves as a weak acid in water due to self ionisation
(b) Acidity of its aqueous solution increases upon addition of ethylene glycol
(c) It has a three-dimensional structure due to hydrogen bonding
(d) It is a weak electrolyte in water
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Answer:
Correct Answer: 14. (b,d)
Solution:
- (a) It does not undergo self ionisation in water but accepts an electron pair from water, so it behaves as weak monobasic acid.
$ \mathrm{H} _{3} \mathrm{BO} _{3}+\mathrm{H} _{2} \mathrm{O} \rightleftharpoons \mathrm{B}(\mathrm{OH}) _{4}^{-}+\mathrm{H}^{+} $
Hence, (a) is incorrect.
(b) When treated with 1, 2-dihydroxy or polyhydroxy compounds, they form chelate (ring complex) which effectively remove $\left[\mathrm{B}(\mathrm{OH}) _{4}\right]$ species from solution and thereby produce maximum number of $\mathrm{H} _{3} \mathrm{O}^{+}$or $\mathrm{H}^{+}$ ions, i.e. results in increased acidity.
(c) Boric acid crystallises in a layer structure in which planar triangular $\mathrm{BO} _{3}^{3–}$ ions are bonded together through hydrogen bonds.
(d) In water the $\mathrm{p} K_{a}$ value of $\mathrm{H} _{3} \mathrm{BO} _{3}$ is 9.25 .
$\mathrm{H} _{3} \mathrm{BO} _{3}+\mathrm{H} _{2} \mathrm{O} \rightleftharpoons \mathrm{B}(\mathrm{OH}) _{4}^{-}+\mathrm{H}^{+} ; \mathrm{p} K _{a}=9.25$
So, it is a weak electrolyte in water.
