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Manipal Engineering Manipal Engineering Solved Paper-2012

  • question_answer
    If\[{{K}_{a}}\]of\[HCN=4\times {{10}^{-10}}\], then the\[pH\]of\[2.5\times {{10}^{-1}}\]molar\[HCN(aq)\]is

    A) 1                                             

    B) 2.5

    C) 4                             

    D)        5

    Correct Answer: D

    Solution :

    \[[{{H}_{3}}{{O}^{+}}]=\sqrt{{{K}_{a}}\times C}\]                 \[=\sqrt{4\times {{10}^{-10}}\times 2.5\times {{10}^{-1}}}\]                 \[=\sqrt{{{10}^{-10}}}={{10}^{-5}}M\] \[\because \]  \[pH=-\log [{{H}_{3}}{{O}^{+}}]\] \[\therefore \]  \[pH=-\log ({{10}^{-5}})=5\]


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