JCECE Engineering JCECE Engineering Solved Paper-2004

  • question_answer
    If \[50%\] of a radioactive substance dissociates in \[15\min \], then the time taken by substance to dissociate \[99%\] will be:

    A) \[50\,\,\min \]                  

    B) \[100\,\,\min \]

    C) \[99\,\,\min \]                  

    D)  \[150\,\,\min \]

    Correct Answer: C

    Solution :

    Key Idea:                 \[k=\frac{2.303}{t}\log \frac{a}{a-x}\] Given,\[t=15,\,\,a=100,\,\,a-x=50\] \[\therefore \]  \[k=\frac{2.303}{15}\log \frac{100}{50}\]                 \[k=\frac{2.303}{15}\log 2\]                                         ? (i) Also\[t=?,\,\,a=100,\,\,a-x=100-99=1\] \[\therefore \]  \[k=\frac{2.303}{t}\log \frac{100}{1}\] or            \[k=\frac{2.303}{t}\log 100\]                       ... (ii) From Eqs. (i) and (ii).                 \[\frac{2.303}{15}\log 2=\frac{2.303}{t}\log 100\] or            \[\frac{1}{15}\log 2=\frac{1}{t}\log 100\] or            \[\frac{0.3010}{15}=\frac{2}{t}\] or            \[t=\frac{15\times 2}{0.3010}\]                    \[=99\min \]


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