BHU PMT BHU PMT (Mains) Solved Paper-2005

  • question_answer
    The half-life of radium is 1620 year and its atomic weight is 226. The number of atoms that will decay from its 1 g sample per second will be:

    A)  \[3.6\times {{10}^{10}}\]            

    B)  \[3.6\times {{10}^{12}}\]

    C)  \[3.1\times {{10}^{15}}\]            

    D)  \[31.1\times {{10}^{15}}\]

    Correct Answer: A

    Solution :

                     According to Avogadro's hypothesis. \[{{N}_{0}}=\frac{6.02\times {{10}^{23}}}{226}=2.66\times {{10}^{21}}\] Half-life\[=T=\frac{0.693}{\lambda }=1620\]year \[\therefore \]  \[\lambda =\frac{0.693}{1620\times 3.16\times {{10}^{7}}}\]                 \[=1.35\times {{10}^{-11}}{{s}^{-1}}\] Because half-life is very much large as compared to its times interval, hence\[N={{N}_{0}}\]. Now,   \[\frac{dN}{dt}=\lambda N=\lambda {{N}_{0}}\] or            \[dN=\lambda {{N}_{0}}dt\]                 \[=(1.35\times {{10}^{-11}})(2.66\times {{10}^{21}})\times 1\]                 \[=1.35\times {{10}^{-11}}{{s}^{-1}}\]


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