JEE Main & Advanced JEE Main Paper (Held On 22 April 2013)

  • question_answer Values of dissociation constant, \[{{\text{K}}_{\text{a}}}\]are given as follows:                 Acid                                        \[{{\text{K}}_{\text{a}}}\]                 \[\operatorname{HCN}\]                                    \[6.2\times {{10}^{-10}}\]                   \[HF\]                                     \[7.2\times {{10}^{-4}}\]                 \[HN{{O}_{2}}\]                                     \[4.0\times {{10}^{-4}}\]                 Correct order of increasing base strength of the base \[{{\operatorname{CN}}^{-}},{{\operatorname{F}}^{-}}\] and \[\operatorname{NO}_{2}^{-}\]will be:     JEE Main  Online Paper (Held On 22 April 2013)

    A)  \[{{\operatorname{F}}^{-}}<{{\operatorname{CN}}^{-}}<\operatorname{NO}_{2}^{-}\]       

    B)  \[NO_{2}^{-}<C{{N}^{-}}<{{F}^{-}}\]

    C)  \[{{\operatorname{F}}^{-}}<\operatorname{NO}_{2}^{-}<{{\operatorname{CN}}^{-}}\]       

    D)  \[\operatorname{NO}_{2}^{-}<\operatorname{F}_{{}}^{-}<\operatorname{CN}_{{}}^{-}\]

    Correct Answer: C

    Solution :

     Higher the value of \[{{K}_{a}}\]lower will be the value of \[p{{K}_{a}}\]i.e. higher will be the acidic nature. Furth since \[\text{CN}_{,}^{-}{{\text{F}}^{-}}\]and \[\text{N}{{\text{O}}_{\text{2}}}^{-}\]are conjugate base of the acids HCN, HF and \[\text{HN}{{\text{O}}_{\text{2}}}\]respectively hence the correct order of base strength will be \[{{F}^{-}}<N{{O}_{2}}^{-}<C{{N}^{-}}\]  (stronger the acid weaker will be its conjugate base)

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