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)

$B(OH)3$

Answer 2

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 $\left( H _{3} BO _{3}\right)$. 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. $B ( OH )_{3}+ H _{2} O \longrightarrow\left[ B ( OH )_{4}\right]^{-}+ H ^{+}$