2003 AIPMT The p-Block ElementsBoric Acid Medium Question 67 of 77 in The p-Block Elements

Which one of the following compounds is not a protonic acid :

A $B(OH)_3$
B $PO(OH)_3$
C $SO(OH)_2$
D $SO_2(OH)_2$

The answer is A.

Solution

A protonic (Brønsted) acid releases a proton ($H^+$) from its own –OH group.
$PO(OH)_3$ (phosphoric acid), $SO(OH)_2$ (sulphurous acid) and $SO_2(OH)_2$ (sulphuric acid) all ionise by releasing $H^+$ from their –OH groups.
Boric acid, $B(OH)_3$, does not release its own proton. Boron has an incomplete octet, so it accepts an $OH^-$ ion from water, which releases $H^+$ from the water:
$B(OH)_3 + H_2O \rightarrow [B(OH)_4]^- + H^+$
So boric acid is a weak monobasic Lewis acid, not a protonic acid.
So the compound that is not a protonic acid is $B(OH)_3$.

Concept Boric acid as a Lewis acid

The p-Block Elements in past papers

77 questions from this chapter have appeared across 23 exam years.