Equilibrium Question 806
Question: 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}} $
HNCX $ 4.0\times {10^{-4}} $
Correct order of increasing base strength of the base $ C{N^{-}}\text{, }{F^{-}} $ and $ NO_2^{-} $ will be:
Options:
A) $ {F^{-}}<C{N^{-}}<NO__2^{-} $
B) $ NO_2^{-}<C{N^{-}}<{F^{-}} $
C) $ {F^{-}}<NO_2^{-}<C{N^{-}} $
D) $ NO_2^{-}<{F^{-}}<C{N^{-}} $
Show Answer
Answer:
Correct Answer: C
Solution:
Higher the value of $ K_{a} $ lower will be the value of $ pK_{a} $ i.e. higher will be the acidic nature.
Further $ C{N^{-}},{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^{-}<C{N^{-}} $ ( $ \therefore $ stronger the acid weaker will be its conjugate base)
