JEE Main & Advanced Sample Paper JEE Main Sample Paper-7

  • question_answer
    A radioactive isotope X with half-life of \[6.93\times {{10}^{9}}\] years decays to Y which is stable. A sample of rock from the moon was found to contain both the elements X and Y in the mole ratio 1 : 7. What is the age of rock?

    A)  \[2.079\times {{10}^{10}}\] years            

    B)  \[1.94\times {{10}^{9}}\] years

    C)  \[2\times {{10}^{15}}\] years    

    D)  \[{{10}^{15}}\] years

    Correct Answer: A

    Solution :

    \[x\,\,\,\,\,\,\,\,\,\to \,\,\,\,\,\,\,\,\,\,\,y\] At time \[t,\,{{n}_{0}}-x\,\,\,\,\,\,\,\,\,\,\,\,x\] \[\frac{{{n}_{0}}-x}{x}=\frac{1}{7}\] \[\Rightarrow \,{{n}_{0}}=\frac{8x}{7}\] \[\lambda =\frac{0.693}{6.93\times {{10}^{9}}}={{10}^{-10}}\] years \[t=\frac{2.303}{{{10}^{-10}}}\log \,8=\frac{2.303}{{{10}^{-10}}}\times 0.9039\] \[=2.079\times {{10}^{10}}\] years


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