CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2008

  • question_answer
    Two radioactive elements X and Fhave half-lives of 6 min and 15 min respectively. An experiment starts with 8 times as many atoms of X as Y. How long it takes for the number of atoms of X left equals the number of atoms of Y left?

    A) 6 min                                    

    B)  12 min

    C) 48 min                  

    D)        30 min

    E)  24 min

    Correct Answer: D

    Solution :

    Let the initial amount of \[Y=1\] So, the initial amount of \[X=8\] X decomposes as \[8\xrightarrow[{}]{1}4\xrightarrow[{}]{2}2\xrightarrow[{}]{3}1\xrightarrow[{}]{{}}\frac{1}{2}\xrightarrow[{}]{5}\frac{1}{4}\] Number of half-lives of \[X=5\] Y decomposes as \[1\xrightarrow[{}]{1}\frac{1}{2}\xrightarrow[{}]{2}\frac{1}{4}\] Number of half-lives of \[Y=2\] So, total time \[T=n\times {{t}_{1/2}}\] \[=5\times 6=30\text{ }min\] or         \[T=n\times {{t}_{1/2}}=2\times 15=30\text{ }min\]


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