Question:

Values of dissociation constant, \(K_a\) are given as follows :
Acid\(K_a\)
HCN\(6.2\times10^{-10}\)
HF\(7.2\times10^{-4}\)
\(\mathrm{HNO_2}\)\(4.0\times10^{-4}\)
Correct order of increasing base strength of the base \(\mathrm{CN^-}\), \(\mathrm{F^-}\) and \(\mathrm{NO_2^-}\) will be :

Show Hint

The stronger the acid (larger \(K_a\)), the weaker its conjugate base: \(K_b = K_w/K_a\).
Updated On: Oct 31, 2026
  • \(\mathrm{F^-} \lt \mathrm{CN^-} \lt \mathrm{NO_2^-}\)
  • \(\mathrm{NO_2^-} \lt \mathrm{CN^-} \lt \mathrm{F^-}\)
  • \(\mathrm{F^-} \lt \mathrm{NO_2^-} \lt \mathrm{CN^-}\)
  • \(\mathrm{NO_2^-} \lt \mathrm{F^-} \lt \mathrm{CN^-}\)
Show Solution
collegedunia
Verified By Collegedunia

The Correct Option is C

Solution and Explanation

\(K_b\) of the conjugate base \(= \frac{K_w}{K_a}\), so a larger \(K_a\) means a smaller \(K_b\).
\(K_a\): \(\mathrm{HF}\) (\(7.2\times10^{-4}\)) \(\gt\) \(\mathrm{HNO_2}\) (\(4.0\times10^{-4}\)) \(\gt\) \(\mathrm{HCN}\) (\(6.2\times10^{-10}\)).
Hence base strength: \(\mathrm{F^-} \lt \mathrm{NO_2^-} \lt \mathrm{CN^-}\).
Answer: (3) \(\mathrm{F^-} \lt \mathrm{NO_2^-} \lt \mathrm{CN^-}\)
Was this answer helpful?
0
0