BHU PMT BHU PMT Solved Paper-2003

  • question_answer
    The pH of a\[{{10}^{-10}}M\text{ }NaOH\]solution is:

    A)  10                                         

    B)  7.01

    C)  6.99                                      

    D)  4

    Correct Answer: B

    Solution :

                     Key Idea: In a very dilute solution, the concentration of\[O{{H}^{-}}\]produced from water cannot be neglected \[\left( \begin{align}   & pOH=-\log [O{{H}^{-}}] \\  & \,\,\,\,pH=14-pOH \\ \end{align} \right)\] \[[O{{H}^{-}}]=\underset{(from\,\,NaOH)}{\mathop{{{10}^{-10}}}}\,+\underset{(from\,water)}{\mathop{{{10}^{-7}}}}\,\] \[={{10}^{-7}}(0.001+1)\] \[=1.001\times {{10}^{-7}}\] \[pOH=-\log [O{{H}^{-}}]\] \[=-\log (1.001\times {{10}^{-7}})\] \[=7-0.01\] \[=6.99\] \[pH=14-pOH\]  \[=14-6.99\] \[=7.01\]


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