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

Hint: 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.