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)



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