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Haryana PMT Haryana PMT Solved Paper-2005

  • question_answer
    The \[_{p}{{K}_{a}}\] of HCN is 9.30. The pH of a solution prepared by mixing \[2.5moles\]of KCN and \[2.5moles\] of HCN in water and making total volume up to 500 ml, is:

    A)  \[9.30\]                              

    B)  \[7.30\]

    C)  \[10.30\]                            

    D)  \[8.30\]      

    Correct Answer: D

    Solution :

                    Cone. of KCN = cone. of HCN \[=\frac{2.5}{500/1000}\] \[=5M\]                 \[\therefore \]  \[\log \frac{[KCN]}{[HCN]}=\log \frac{5}{5}=0\] Now, from Hendersons equation,                                 \[pH{{=}_{p}}{{K}_{a}}+\log \frac{[salt]}{[acid]}\]                                 \[=9.30+0=9.30\]


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