Question

Which of the following is only acidic in nature?

• A. $Be(OH{)}_2$
• B. $Mg(OH{)}_2$
• C. $B(OH)3$
• D. $Al(OH)3$

Answer 1

Answer: (C)

Among the given compounds $Mg(OH{)}_2$ is weakly basic, $Al(OH{)}_3$ and $Be(OH{)}_2$ are amphoteric. Only $B(OH{)}_3$ is acid. It is a weak monobasic acid - Boric acid $\left(H_{3}B{O}_3\right)$. Also it is Lewis acid and not a proton donar (Bronsted acid). $B(OH{)}_3$ accept lone pair of electrons from water molecule
.
In this species $B^{3+}$ due to its very small size polarize $O$-atom upto greater extent and attracts its electron cloud towards itself.
Hence, $O$ atom in turn pull the bonding electrons of $O-H$ bond thereby making this bond weak. Thus this species has tendency to loose proton.
$B(OH{)}_3+H_{2}O⟶{\left[B(OH{)}_4\right]}^{-}+H^{+}$