Values of dissociation constant, $\mathrm{K}_{\mathrm{a}}$ are given as follows: $\begin{array}{ll}\text {…

Values of dissociation constant, $\mathrm{K}_{\mathrm{a}}$ are given as follows:
$\begin{array}{ll}\text { Acid } & \mathrm{K}_{\mathrm{a}} \\ \mathrm{HCN} & 6.2 \times 10^{-10} \\ \mathrm{HF} & 7.2 \times 10^{-4} \\ \mathrm{HNO}_{2} & 4.0 \times 10^{-4}\end{array}$
Correct order of increasing base strength of the base $\mathrm{CN}^{-}, \mathrm{F}^{-}$ and $\mathrm{NO}_{2}^{-}$ will be :
  1. $\mathrm{F}^{-} < \mathrm{CN}^{-} < \mathrm{NO}_{2}^{-}$
  2. $\mathrm{NO}_{2}^{-} < \mathrm{CN}^{-} < \mathrm{F}^{-}$
  3. $\mathrm{F}^{-} < \mathrm{NO}_{2}^{-} < \mathrm{CN}^{-}$
  4. $\mathrm{NO}_{2}^{-} < \mathrm{F}^{-} < \mathrm{CN}^{-}$

Solution

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 $\mathrm{CN}^{-}, \mathrm{F}^{-}$ 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) ~

Asked in: JEE-TOPICTESTS-CHEMISTRY

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