Question

Assertion :In water, orthoboric acid behaves as a weak monobasic acid. Reason: In water, orthoboric acid acts as a proton donor.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is C Assertion is correct but Reason is incorrect

Ortho boric acid does not donate proton, like most of the acids but rather it accepts $O{H}^{-}$, therefore it is a lewis acid and is represented by $B(OH{)}_{3^{-}}$.

$B(OH{)}_3+2H_{2}O\to H_3{O}^{+}+[B\left(OH{)}_4{]}^{-}\right(P_{k_a}=9.25)$

$B(OH{)}_3$ partially reacts with water to form $H_3{O}^{+}$ and $B\left(OH{)}_4^{-}\right]$ and behaves as a weak acid.

Thus $B(OH{)}_3$ can not be titrated satisfactorily with $NaOH$ because a sharp end point is not obtained. If polyhydroxy compound like glycerol, mannitol or sugar are added to titration mixture than it can be titrated with $NaOH$.

$B(OH{)}_3+NaOH\to Na[B\left(OH{)}_4\right]$.

Assertion is correct but Reason is incorrect