Values of dissociation constant, $K_a$ are given as follows : $\begin{array}{lc}\text { Acid } &…
Values of dissociation constant, $K_a$ are given as follows :
$\begin{array}{lc}\text { Acid } & \boldsymbol{K}_{\boldsymbol{a}} \\ \mathrm{HCN} & 6.2 \times 10^{-10} \\ \mathrm{HF} & 7.2 \times 10^{-4} \\ \mathrm{HNO}_2 & 4.0 \times 10^{-4}\end{array}$
Correct order of increasing base strength of the base $\mathrm{CN}^{-}, \mathrm{F}^{-}$and $\mathrm{NO}_2^{-}$will be :
Higher the value of $\mathrm{K}_a$ lower will be the value of $\mathrm{pK}_{\mathrm{a}}$ i.e. higher will be the acidic nature. Further since $\mathrm{CN}^{-}, \mathrm{F}^{-}$and $\mathrm{NO}_2^{-}$are conjugate base of the acids $\mathrm{HCN}, \mathrm{HF}$ and $\mathrm{HNO}_2$ respectively hence the correct order of base strength will be
$
\mathrm{F}^{-} < \mathrm{NO}_2^{-} < \mathrm{CN}^{-}
$
$(\because$ stronger the acid weaker will be its conjugate base)