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BCECE Engineering BCECE Engineering Solved Paper-2013

  • question_answer
    \[p{{K}_{b}}\]of \[\text{N}{{\text{H}}_{\text{3}}}\]is 4.74 and \[\text{p}{{\text{K}}_{b}}\]of \[{{A}^{-}},{{B}^{-}}\]and \[{{C}^{-}}\] are 4,5 and 6 respectively. Aqueous solution of 0.01 M has pH in the increasing order

    A) \[N{{H}_{4}}A<N{{H}_{4}}B<N{{H}_{4}}C\]

    B) \[N{{H}_{4}}C<N{{H}_{4}}B<N{{H}_{4}}A\]

    C) \[N{{H}_{4}}C<N{{H}_{4}}A<N{{H}_{4}}B\]

    D) All have equal pH

    Correct Answer: B

    Solution :

    \[\because \]  Salts \[N{{H}_{4}}A,\,N{{H}_{4}}B,\,N{{H}_{4}}C\]are of weak acid and weak base.                 \[\therefore \]  \[pH=7+\frac{p{{K}_{a}}}{2}-\frac{p{{K}_{a}}}{2}\] Thus, greater the value of \[p{{K}_{a}}\] of HA, greater the pH.                 \[p{{K}_{b}}({{A}^{-}},{{B}^{-}},{{C}^{-}})|\underset{4}{\mathop{{{A}^{-}}}}\,\underset{5}{\mathop{<{{B}^{-}}}}\,<\underset{6}{\mathop{{{C}^{-}}}}\,\]                 \[\therefore \] \[pKa(HA,\,HB,\,HC)|\underset{8}{\mathop{HC}}\,\underset{9}{\mathop{<HB}}\,\underset{10}{\mathop{\,<HA}}\,\] Thus, order of pH values is \[(N{{H}_{4}}C<N{{H}_{4}}B<N{{H}_{4}}A).\]


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