# Basicity of anions

Which one of the following is strongest base?

A. $$\ce{Cl-}$$
B. $$\ce{Br-}$$
C. $$\ce{I-}$$
D. $$\ce{CH3COO-}$$

Someone please tell me the answer with proper reason. First I thought that it's $$\ce{Cl-},$$ but then it clicks in my mind that it's $$\ce{CH3COO-}$$ since it's a conjugate base of a weak acid.

You actually got it! The proper way to get to the (right) answer is to ask yourself which one is the conjugate base of a weak acid. If you are given more than one weak acid, you should be provided with the respective acid dissociation constants $$K_\mathrm{a}$$. In that case, keep in mind that the smaller (i.e. the closer to zero) the $$K_\mathrm{a},$$ the stronger the acid.