JEE Main & Advanced Sample Paper JEE Main Sample Paper-8

  • question_answer
    A sample of \[{{U}^{238}}({{t}_{1/2}}=4.5\times {{10}^{9}}yrs)\]is found to contain \[11.9gm\text{ }{{U}^{238}}\]and \[20.6gm\text{ }P{{b}^{206}}.\]The age of sample is - (Given In\[2=0.693,\ln 3=1.1\]]

    A)  \[1.64\times {{10}^{10}}\text{ }yrs\]                     

    B)  \[7.14\times {{10}^{9}}\text{ }yrs\]

    C)  \[2.25\times {{10}^{9}}\text{ }yrs\]        

    D)  None of these

    Correct Answer: B

    Solution :

     \[t=\frac{1}{\lambda }\ln \frac{{{n}_{U}}+{{n}_{Pb}}}{{{n}_{U}}};{{n}_{U}}=no.\]of mole of U238 \[t=\frac{1}{\lambda }\ln \left( \frac{0.15}{0.05} \right);{{n}_{Pb}}=no.\] of mole of Pb206 \[t=\frac{1}{\lambda }\ln (3)=\frac{1.1}{0.693}\times 4.5\times {{10}^{9}}=7.14\times {{10}^{9}}\]years


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