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JEE Main 201322 AprilOnline
Exam
JEE Main
Year
2013
Date
22 April
Mode
Online
Subject
Chemistry
Chapter
Ionic Equilibrium

JEE Main 2013 · 22 April · Online · Chemistry · Ionic Equilibrium

Values of dissociation constant, K_a are given as follows : array...

Values of dissociation constant, KaK_a are given as follows : $\begin{array}{lc}\text{ Acid } & \boldsymbol{K}_{\boldsymbol{a}} \\ \mathrm{HCN} & 6.2 \times 10^{-10} \\ \mathrm{HF} & 7.2 \times 10^{-4} \\ \mathrm{HNO}_2 & 4.0 \times 10^{-4}\end{array} Correct order of increasing base strength of the base CN−\mathrm{CN}^{-},,F−\mathrm{F}^{-}andandNO2−\mathrm{NO}_2^{-}$will be :

Options

  1. AF−<CN−<NO2−\mathrm{F}^{-} < \mathrm{CN}^{-} < \mathrm{NO}_2^{-}
  2. BNO2−<CN−<F−\mathrm{NO}_2^{-} < \mathrm{CN}^{-} < \mathrm{F}^{-}
  3. CF−<NO2−<CN−\mathrm{F}^{-} < \mathrm{NO}_2^{-} < \mathrm{CN}^{-}
  4. DNO2−<F−<CN−\mathrm{NO}_2^{-} < \mathrm{F}^{-} < \mathrm{CN}^{-}

Correct answer

C. F−<NO2−<CN−\mathrm{F}^{-} < \mathrm{NO}_2^{-} < \mathrm{CN}^{-}

Step-by-step solution

Higher the value of Ka\mathrm{K}_a lower will be the value of pKa\mathrm{pK}_{\mathrm{a}} i.e. higher will be the acidic nature. Further since CN−,F−\mathrm{CN}^{-}, \mathrm{F}^{-}and NO2−\mathrm{NO}_2^{-}are conjugate base of the acids HCN,HF\mathrm{HCN}, \mathrm{HF} and HNO2\mathrm{HNO}_2 respectively hence the correct order of base strength will be F−<NO2−<CN−\mathrm{F}^{-} < \mathrm{NO}_2^{-} < \mathrm{CN}^{-} (∵(\because stronger the acid weaker will be its conjugate base)
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