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Q.
The correct statement(s) for orthoboric acid is/'are
IIT JEEIIT JEE 2004The p-Block Elements
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.
