JEE Challenger
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Acidic Behavior of Orthoboric Acid in Water

STATEMENT-1 : In water, orthoboric acid behaves as a weak monobasic acid.
because
STATEMENT-2 : In water, orthoboric acid acts as a proton donor.

Options

A

Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1

B

Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1

C

Statement-1 is True, Statement-2 is False

Correct
D

Statement-1 is False, Statement-2 is True

Step-by-Step Solution

To determine the correct option, let us analyze both statements:

  1. Analysis of STATEMENT-1:
    Orthoboric acid, represented as H3BO3\text{H}_3\text{BO}_3 or B(OH)3\text{B(OH)}_3, has a central boron atom surrounded by six valence electrons, which leaves its octet incomplete. In an aqueous solution, it acts as a Lewis acid by accepting a lone pair of electrons from the oxygen atom of a hydroxide ion (OH−\text{OH}^-) derived from a water molecule:
    B(OH)3+2H2O⇌[B(OH)4]−+H3O+\text{B(OH)}_3 + 2\text{H}_2\text{O} \rightleftharpoons [\text{B(OH)}_4]^- + \text{H}_3\text{O}^+
    Because each mole of B(OH)3\text{B(OH)}_3 results in the liberation of one mole of hydronium ions (H3O+\text{H}_3\text{O}^+), and the equilibrium constant is small (Ka≈5.8×10−10K_a \approx 5.8 \times 10^{-10}), it behaves as a weak monobasic acid.
    Therefore, STATEMENT-1 is True.

  2. Analysis of STATEMENT-2:
    A Brønsted-Lowry acid acts directly as a proton donor by ionizing to release H+\text{H}^+ from its own molecular structure. Orthoboric acid does not donate its own protons; instead, it behaves as a Lewis acid by accepting an OH−\text{OH}^- ion from water, causing the water molecule to release a proton (H+\text{H}^+).
    Therefore, orthoboric acid acts as a Lewis acid (electron-pair acceptor) rather than a proton donor.
    Therefore, STATEMENT-2 is False.

Since Statement-1 is True and Statement-2 is False, the correct choice is (C).