Question

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

Answer 1

Hint: Basicity of an acid depends upon the number of protons it can give upon its dissolution in an aqueous medium. Boric acid contains three –OH groups in its structure.

Complete step by step solution:
Let’s see which type of acid ${H_3}B{O_3}$ (Boric acid) is.
-Basicity of the acids is the number of protons an acid can give upon dissociation in an aqueous solution. So, if an acid can give one proton upon dissociation in aqueous solution, then the acid is said to be monoprotic. Thus, a diprotic acid will give two protons upon dissociation in aqueous solution.
-The chemical structure of boric acid and its reaction in an aqueous medium can be given as

-Here, we can say that though it has three protons, it will not act as a proton donor. It will react with water molecules to form ${[B{(OH)_4}]^ - }$ and will give one proton. So, in this way, it will act as an acid.
-Here, we can see that one molecule of boric acid gives one proton. So, this acid can be considered as a monoprotic acid.
-Bronsted acid is an acid which is capable of donating a proton from its molecule. Boric acid is not able to donate the proton. So, it is not a Bronsted acid.
-Lewis acid is an acid that can accept electron pairs from the base. Here Boric acid can accept the electron pair from water molecules as shown in the reaction. Thus, Boric acid is a Lewis acid.
So, we can conclude that Boric acid is a monoprotic and Lewis acid.

Therefore, the correct answer is (A).

Note: The most common mistake we make here is that we consider Boric acid as a triprotic acid. Actually, it is a Lewis acid also and so that it will form a complex ${[B{(OH)_4}]^ - }$ with water. As this complex is formed, it is not able to donate three protons and actually, it is able to donate only one proton.