VIT Engineering VIT Engineering Solved Paper-2014

  • question_answer
    The activity of a radioactive sample is measured as \[{{N}_{0}}\]counts per minute at t = 0 and \[{{N}_{0}}/C\] counts per minute at t= 5 min. The time, (in minute) at which the activity reduces to half its value, is

    A) \[{{\log }_{e}}\frac{2}{5}\]                          

    B) \[\frac{5}{{{\log }_{e}}2}\]

    C) \[5{{\log }_{10}}2\]                         

    D) \[5{{\log }_{e}}2\]

    Correct Answer: D

    Solution :

    Fraction remains after n half-lives                   \[2NaOH+C{{O}_{2}}\to N{{a}_{2}}C{{O}_{3}}+{{H}_{2}}O\]                  \[N{{a}_{2}}C{{O}_{3}}+2HCl\to 2NaCl+{{H}_{2}}O+C{{O}_{2}}\]                 \[{{p}^{5+}}.\] Taking log on both sides, we get           \[8Al\to 8A{{l}^{3+}}+24{{e}^{-}}\]                       \[9F{{e}^{8/3+}}+24{{e}^{-}}\to 9Fe\]                          \[\underset{\left( \text{no lp} \right)}{\mathop{NH_{4}^{+}}}\,>\underset{\left( 1lp \right)}{\mathop{N{{H}_{3}}}}\,>\underset{\left( 2lp \right)}{\mathop{NH_{2}^{-}}}\,\] Now, let t? be the time after which activity reduces to half                      \[\frac{n{{'}_{He}}}{n{{'}_{C{{H}_{4}}}}}=\frac{1}{2}\sqrt{\frac{16}{4}}=\frac{1}{1}\]                       \[\frac{n{{'}_{He}}}{n{{'}_{S{{O}_{2}}}}}=\frac{1}{3}\sqrt{\frac{64}{4}}=\frac{4}{3}\]


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