Question

Which one of the following ionic species has the greatest proton affinity to form stable compound?

• A. $I^{-}$
• B. $H{S}^{-}$
• C. $N{H}_2^{-}$
• D. $C{l}^{-}$

Answer 1

The correct option is C

$N{H}_2^{-}$

In other words, the question wants us to figure out the strongest base from the given anions. Also, the strongest base will have the weakest corresponding conjugate acid. The easier way is to compare the pKa values: ammonia has the highest pKa (weakest acid) of 36 and hence, $N{H}_2^{-}$ is the strongest base.