NEET Sample Paper NEET Sample Test Paper-87

  • question_answer
    The rapid change of pH near the stoichiometric point of an acid-base titration is the basis of indicator detection. pH of the solution is related to ratio of the concentrations of the conjugate acid (HIn) and base \[(I{{n}^{-}})\] forms of the indicator by the expression

    A) \[\log \frac{\left[ I{{n}^{-}} \right]}{\left[ HIn \right]}=p{{K}_{In}}-pH\]

    B) \[\log \frac{\left[ HIn \right]}{\left[ I{{n}^{-}} \right]}=p{{K}_{In}}-pH\]

    C) \[\log \frac{\left[ HIn \right]}{\left[ I{{n}^{-}} \right]}=pH-p{{K}_{In}}\]

    D) \[\log \frac{\left[ I{{n}^{-}} \right]}{\left[ HIn \right]}=pH-p{{K}_{In}}\]

    Correct Answer: D

    Solution :

    [d]  For an acid-base indicator \[HIn{{H}^{+}}+I{{n}^{-}}\] \[\therefore \]\[{{K}_{In}}=\frac{[{{H}^{+}}][I{{n}^{-}}]}{[HIn]}\] or \[[{{H}^{+}}]={{K}_{In}}\times \frac{[HIn]}{[I{{n}^{-}}]}\] or \[log\,{{H}^{+}}=\log {{K}_{In}}+\log \frac{[HIn]}{[I{{n}^{-}}]}\] Taking negative on both sides \[-log[{{H}^{+}}]=-\log {{K}_{In}}-\log \frac{[HIn]}{[I{{n}^{-}}]}\] or we can write \[pH=p{{K}_{In}}+\log \frac{[I{{n}^{-}}]}{[HIn]}\] or \[\log \frac{[I{{n}^{-}}]}{[HIn]}=pH-p{{K}_{In}}\]


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