Question

Among the following, the species that act as the weakest base is:

• A. $O{H}^{-}$
• B. $C{H}_3^{-}$
• C. $C{H}_3{O}^{-}$
• D. $C{l}^{-}$

Answer 1

The correct option is D $C{l}^{-}$

Species formed by the gain of a proton from a base is called conjugate acid of that base.
Conjugate acids of all the species are:
A. $H^{+}+H^{-}\rightleftharpoons H_2$
B. $H^{+}+C{H}_3^{-}\rightleftharpoons C{H}_4$
C. $H^{+}+C{H}_3{O}^{-}\rightleftharpoons C{H}_{3}OH$
D. $H^{+}+C{l}^{-}\rightleftharpoons HCl$
Among the conjugate acids, $H_2,C{H}_4,C{H}_{3}OHandHCl$
$HCl$ is the strongest conjugate acid, so the base it is formed from is the weakest base i.e. it has the lowest ability to accept proton because of the strong acidic nature of $HCl$.
Hence, $C{l}^{-}$ is the weakest base.