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JEE Main & Advanced Chemistry Surface & Nuclear Chemistry / भूतल और नाभिकीय रसायन Question Bank Rate of decay and Half-life

  • question_answer
    A radioactive isotope has a half-life of 10 days. If today \[125\,mg\]is left over, what  was its original weight 40 days earlier                [KCET 2005]

    A)                 2g          

    B)                 600 mg

    C)                 1 g         

    D)                 1.5 g

    Correct Answer: A

    Solution :

           \[N={{N}_{0}}{{\left( \frac{1}{2} \right)}^{n}}\]. \[n=\frac{40}{10}=4\]                                 \[\frac{125}{1000}={{N}_{0}}{{\left( \frac{1}{2} \right)}^{4}},\,{{N}_{0}}=\frac{125}{1000}\times 2\times 2\times 2\times 2=2g\]


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