JEE Advanced 2014 · Paper 1 · Chemistry Q28

ChemistryMultiple correct+3Question 28 of 60

The correct statement(s) for orthoboric acid is/are

It behaves as a weak acid in water due to self ionization.

Acidity of its aqueous solution increases upon addition of ethylene glycol.

It has a three dimensional structure due to hydrogen bonding.

It is a weak electrolyte in water.

Answer and worked solution

Official answer: B, D

(A) Boric acid is a weak monobasic Lewis acid: it accepts $\ce{OH-}$ from water, $\ce{B(OH)3 + 2H2O <=> [B(OH)4]- + H3O+}$ . It does not ionise by itself. False.
(B) Ethylene glycol (a cis-diol) forms a stable chelate with $\ce{[B(OH)4]-}$ , pulling the equilibrium right. The acidity rises enough for titration with NaOH. True.
(C) Hydrogen bonding links the planar $\ce{B(OH)3}$ units into flat layers: a two-dimensional sheet structure. False.
(D) It ionises only slightly, so it is a weak electrolyte. True.

(B), (D)

Topics:Group 13 Acids and bases

JEE Advanced 2014 · Paper 1, Chemistry question 28