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Q)

# $H_3BO_3$ is

(a) monobasic and weak Lewis acid

(b) monobasic and weak Bronstead acid

(c) monobasic and strong Lewis acid.

(d) tribasic and weak Bronstead acid

Comment
A)
Answer: monobasic and weak Lewis acid

$H_3BO_3$ in water gives $H^+$ ions as follows
$H_3BO_3 + H_2O \longrightarrow [Ba(OH)_4]^- + H^+$
It is monobasic as one molecule of the acid gives one $H^+$ ion. As $H_3BO_3$ or $B(OH)_3$ takes up a pair of electrons from $OH^-$, it is a Lewis acid.

Comment
A)
Solution :
The central boron atom in boric acid, H3BO3 is electron−deficient i.e. boric acid is a Lewis acid with one p−orbital vacant. There is no d−orbital of suitable energy in boron atom.
So, it can accommodate only one additional electron pair in its outermost shell.