p-Block Elements-I - Result 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. $(a, b, c)$
Solution:
- (a) It does not undergo self ionisation in water but accepts an electron pair from water, so it behaves as weak monobasic acid.
$$ H _3 BO _3+H _2 O \rightleftharpoons B(OH) _4^{-}+H^{+} $$
Hence, (a) is incorrect.
(b) When treated with 1, 2-dihydroxy or polyhydroxy compounds, they form chelate (ring complex) which effectively remove $\left[B(OH) _4\right]$ species from solution and thereby produce maximum number of $H _3 O^{+}$or $H^{+}$ ions, i.e. results in increased acidity.
(c) Boric acid crystallises in a layer structure in which planar triangular $BO _3^{3–}$ ions are bonded together through hydrogen bonds.
(d) In water the $p K _a$ value of $H _3 BO _3$ is 9.25 .
$H _3 BO _3+H _2 O \rightleftharpoons B(OH) _4^{-}+H^{+} ; p K _a=9.25$
So, it is a weak electrolyte in water.
