Question

Which of the following is an acid?
A. $B{(OH)_3}$
B. $In{(OH)_3}$
C. Both A and B
D. None of the above

Answer 1

Generally, basicity decreases, and acidity increases as we move down the group in the periodic table. As we move down the group, the size increases and it will lead to a decrease in effective nuclear charge. Effective nuclear charge is the attraction of nucleus and valence shell electrons. The decrease in effective nuclear charge results in increased acidity of the given molecule.

Complete step by step answer:
The elements in group-13 are Boron, Aluminium, Gallium, Indium, and Thallium. The general valence electronic configuration for group-13 is $n{s^2}n{p^1}$. The acidity nature of hydroxides decreases down the group.
The molecular formula for boric acid is $B{(OH)_3}$. All the elements except boron in group-13 are metallic. Thus, boron can't give stable cation and thus, it acts as non-metallic. And as a result, boron hydroxide tends to give protons even though it has three hydroxyl groups. Thus, boron is a weak acid and also it is a monobasic acid since it readily accepts one hydroxyl ion to form $B{(OH)_4}^ -$. Boric acid is also said to be Lewis acid since it acts as the electron acceptor.
It is known that basicity decreases and acidity increases as we move down the group in the periodic table. Thus, $In{(OH)_3}$ is amphoteric. Amphoteric means the given compound acts as acid as well as the base.
Thus, $B{(OH)_3}$ is acid.

So, the correct answer is Option A.

Note: Aluminium hydroxide and gallium hydroxide also act as amphoteric. Thallium (iii) hydroxide acts as a weak base whereas thallium (i) hydroxide acts as a strong base. Boric acid is a white crystalline solid. And it mainly acts as an antiseptic.