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6.2. Equilibrium - II (icon Equilibrium) — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry6.2. Equilibrium - II (icon Equilibrium)1 Mark
MCQ
Values of dissociation constant, $K_a$ are given as follows

      Acid       $K_a$
      $HCN$       $6.2\times 10^{-10}$
      $HF$       $7.2\times 10^{-4}$
      $HNO_2$       $4.0\times 10^{-4}$

Correct order of increasing base strength of the base $CN^-,F^-$ and $NO_2^-$ will be

  • A
    $F^- < CN^- < NO_2^-$
  • B
    $NO_2^- < CN^- < F^-$
  • ✓
    $F^- < NO_2^- < CN^-$
  • D
    $NO_2^- < F^- < CN^-$
✓

Answer

Correct option: C.
$F^- < NO_2^- < CN^-$
c
Higher the value of $K_a$ lower will be the value of $pK_a$ $i.e.$ higher will be the acidic nature. Further since $CN^-, F^-$ and $NO_2^-$ are conjugate base of the acids $HCN, HF$ and $HNO_2$ respectively hence the correct order of base strength will be $F^- < NO_2^- < CN^-$ ( $\because $ stronger the acid weaker will be its conjugate base)

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