Question

${{H}_{3}}B{{O}_{3}}$ is
(A) Monobasic acid and weak Lewis acid
(B) Monobasic acid and weak Bronsted acid
(C) Monobasic and strong Lewis acid
(D) Tribasic and weak Bronsted acid

Answer 1

Any acid can be defined as monobasic or dibasic or tribasic depending upon its capacity of giving ${{H}^{+}}$ion. If it gives only one proton, then it is monobasic.

Complete answer:
- ${{H}_{3}}B{{O}_{3}}$ is also known as Boric acid or Orthoboric acid. It exists in the form of colourless crystals or a white powder that dissolves in water.
- The property of accepting or donating $O{{H}^{-}}$ ion determines what kind of acid it is. Since it accepts $O{{H}^{-}}$ion, it is a weak Lewis acid.
- Although Boric acid contains 3 OH groups yet it can act as monobasic acid rather than tribasic acid. This is because it does not act as a proton donor rather it accepts a pair of electrons from OH- ions.
- This can be illustrated by the following equation
$${{H}{3}}B{{O}{3}}+{{H}{2}}O\to B{{(O{{H}^{-}})}{4}}+{{H}^{+}}$\- As per the above equation, water splits into${{H}^{+}}$and$O{{H}^{-}}$ions.$O{{H}^{-}}$bonds with$B{{(OH)}{3}}$forming$B{{(O{{H}^{-}})}{4}}$while${{H}^{+}}$ion is being released in the solution. \- Since, only one${{H}^{+}}$$can be released by a water molecule, boric acid is a monobasic acid.

Therefore, the answer to the above question is (A) Monobasic acid and weak Lewis acid.

Additional information: It is often used as an antiseptic, antifungal, insecticide, flame retardant. Boric acid is toxic for the human body, causing skin and eye irritation.

Note: The important thing to notice here is that the ${{H}^{+}}$ ion comes from water, instead of $B{{(OH)}_{3}}$. Boric acid does not dissociate to produce ions rather it forms metaborate ions and in turn releases ions.