BCECE Medical BCECE Medical Solved Papers-2013

  • question_answer
    Equilibrium constant for the reaction, \[N{{H}_{4}}OH+{{H}^{+}}NH_{4}^{+}+{{H}_{2}}O\] is \[1.8\times {{10}^{9}}\]. Hence, equilibrium constant for\[N{{H}_{3}}\,\,(aq)+{{H}_{2}}ONH_{4}^{+}+O{{H}^{-}}\] is

    A)  \[1.8\times {{10}^{-5}}\]   

    B)  \[1.8\times {{10}^{5}}\]

    C)  \[1.8\times {{10}^{-9}}\]

    D)  \[5.59\times {{10}^{-10}}\]

    Correct Answer: A

    Solution :

    \[N{{H}_{4}}OH+{{H}^{+}}NH_{4}^{+}+{{H}_{2}}O\] \[\therefore \] \[K=\frac{[NH_{4}^{+}]\,[{{H}_{2}}O]}{[N{{H}_{4}}OH]\,[{{H}^{+}}]}=1.8\times {{10}^{9}}\] Again, \[N{{H}_{3}}+{{H}_{2}}O\xrightarrow{{}}N{{H}_{4}}OHNH_{4}^{+}+O{{H}^{-}}\] \[\therefore \] \[K=\frac{[NH_{4}^{+}]\,[O{{H}^{-}}]}{[N{{H}_{4}}OH]}\] Now      \[\frac{K}{K}=\frac{[NH_{4}^{+}]\,[O{{H}^{-}}]}{[N{{H}_{4}}OH]}\times \frac{[N{{H}_{4}}OH]\,[{{H}^{+}}]}{[NH_{4}^{+}]\,[{{H}_{2}}O]}\]                 \[=\,[O{{H}^{-}}]\,\,[{{H}^{+}}]\] (\[\because {{H}_{2}}O\] is in excess)                 \[={{K}_{w}}=1\times {{10}^{-14}}\] \[\therefore \] \[K=K\times 1\times {{10}^{-14}}\] \[=1.8\times {{10}^{9}}\times {{10}^{-14}}=1.8\times {{10}^{-5}}\]


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