Question

${H_3}B{O_3}$ is:
(A) Monobasic and weak Lewis acid
(B) Monobasic and weak Bronsted acid
(C) Monobasic and Lewis acid
(D)Tribasic and weak Bronsted acid

Answer 1

Hint: Boron forms several oxyacids and ${H_3}B{O_3}$ is one among them. The acidic nature of a compound is the ability of the compound to donate ${H^ + }$ ions easily when it is in solution.

Complete step by step solution:
The chemical name of ${H_3}B{O_3}$ is Tetrahydroxydiboron which is one of the oxyacids of boron. The chemical species that can accept electron pairs are called Lewis acids and the chemical species that can donate electron pairs are called Lewis bases. It is very important to know the Lewis theory acid-base reaction for better understanding of the concept behind this question. The Lewis theory of acid-base reaction is the reaction between a Lewis acid, which is an electron pair acceptor and a Lewis base, which is an electron-pair donor resulting in the formation of the product where the constituent molecules are held together by a coordinate covalent bond. The difference between monobasic acid, dibasic acid and tribasic acid are the acids that furnish one ${H^ + }$ ion per molecule in water, then it is known as monobasic acid whereas, dibasic acids are the acid that furnishes two ${H^ + }$ ions per molecule in water and the acids the furnishes three ${H^ + }$ ions per molecule in water is known as tribasic acids.
${H_3}B{O_3}$ is not a protonic acid but behaves as a weak Lewis acid since it accepts a pair of electrons from $O{H^ - }$. The $O{H^ - }$ is obtained from ${H_2}O$. For better understanding let's write the chemical reaction for this:
\[{H_2}O\overset {} \leftrightarrows {H^ + } + O{H^ - }\] ………………………… equation 1
\[{(OH)_3}B + O{H^ - }\overset {} \leftrightarrows B(OH)_4^ - \]……………………equation 2
Adding equation 1 and 2
\[{H_3}B{O_3} + {H_2}O \to B(OH)_4^ - + {H^ + }\]…………………..equation 3
Observe equation 2 carefully, this equation follows the Lewis theory of acid-base. From equation 3 we can conclude that ${H_3}B{O_3}$ is monobasic as it furnishes only one ${H^ + }$ ion per molecule in water.

Thus option (A) is the correct answer. ${H_3}B{O_3}$ is monobasic and weak Lewis acid.

Note: Equation 2 exactly meets the Lewis theory of acid-base, here ${(OH)_3}B$ acts as Lewis acid and $O{H^ - }$ acts as Lewis base resulting in the formation of $B(OH)_4^ - $ which has coordinate covalent bond between them $[B(OH)_3^ - \xleftarrow{{}}O{H^ - }]$.