NEET Physics Nuclear Physics And Radioactivity NEET PYQ-Nuclear Physics

  • question_answer
    Two radioactive nuclei P and Q, in a given sample decay into a stable nucleous R. At time \[t=0,\] number of P species are \[4\,{{N}_{0}}\] and that of Q are \[{{N}_{0}}\] Half-life of P (for conversion to K) is 1 min where as that of Q is 2 min. Initially there are no nuclei of R present in the sample. When number of nuclei of P and Q are equal, the number of nuclei of R present in the sample would be                                                                                                    [AIPMT (M) 2011]

    A)  \[3{{N}_{0}}\]            

    B)  \[\frac{9{{N}_{0}}}{2}\]

    C)  \[\frac{5{{N}_{0}}}{2}\]         

    D)       \[2{{N}_{0}}\]

    Correct Answer: B

    Solution :

    Initially \[P\to 4{{N}_{0}}\]
                \[Q\to {{N}_{0}}\]
       Half life          \[{{T}_{p}}\to 1\,\min \]
       \[{{T}_{Q}}\to 2\,\min \]
    Let after time t number of nuclei of P and Q are equal i.e., \[\frac{4{{N}_{0}}}{{{2}^{t/1}}}=\frac{{{N}_{0}}}{{{2}^{t/2}}}\]
       \[4={{2}^{t/2}}\]
       \[{{2}^{2}}={{2}^{t/2}}\]
    \[\frac{t}{2}=2\]
       \[t=4\,\min \] 
    Disactive nucleus or Nuclei of R
                \[=\left( 4{{N}_{0}}-\frac{4{{N}_{0}}}{{{2}^{4}}} \right)+\left( {{N}_{0}}-\frac{{{N}_{0}}}{{{2}^{2}}} \right)\]
                \[=4{{N}_{0}}-\frac{{{N}_{0}}}{4}+{{N}_{0}}-\frac{{{N}_{0}}}{4}\]
                \[=5{{N}_{0}}-\frac{{{N}_{0}}}{2}\]
                \[=\frac{9}{2}{{N}_{0}}\]


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