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Chemical Kinetics Radioactivity

  1. Sep 17, 2012 #1


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    1. The problem statement, all variables and given/known data
    The mean life of a certain radioactive element is 6 hrs. By what fraction will its initial activity decrease over a time period of 5 hrs.

    2. Relevant equations
    Initial Activity = kN[itex]_{0}[/itex]
    Final Activity = kN[itex]_{t}[/itex]

    3. The attempt at a solution
    From eqn 1 I obtain the value of k = 0.166 hr[itex]^{-1}[/itex]
    Since I have to calculate the fraction of initial and final activity
    Dividing eqn 2 by 3
    [itex]\frac{Initial Activity}{Final Activity}[/itex]=[itex]\frac{N_{0}}{N_{t}}[/itex]
    So this means now I have to calculate [itex]\frac{N_{0}}{N_{t}}[/itex] which will be my answer

    So using first order kinetics equation


    Plugging the known values I get [itex]\frac{N_{0}}{N_{t}}[/itex]=2.3006
    which is unfortunately incorrect. But how?
  2. jcsd
  3. Sep 18, 2012 #2
    What happened to the 0.166 /hr which you went to so much trouble to calculate?

    And remember that the factor 2.303 only applies for logs with base 10; for natural logs and exponentials it disappears.

    And finally, the question "by what fraction will it decrease?" is ambiguous. The wording might mean "what fraction of the initial activity is the final activity?" or "by what fraction of the initial activity has the final activity decreased?" To give an example, if the final activity was found to be 25% of the initial activity, was the questioner really expecting the answer 25% or 75%? But in either case, an answer greater than 1 does not make sense.
  4. Sep 19, 2012 #3


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    Yes I also agree with you but the correct answer is itself greater than 1.
  5. Sep 19, 2012 #4
    OK, so the ambiguity has a third and a fourth string:

    Interpretation 1: (final activity)/(initial activity)
    Interpretation 2: (Initial activity – final activity)/(Initial activity)
    Interpretation 3: (Initial activity – final activity)/(final activity)
    Interpretation 4: (Initial activity)/(final activity)

    The problem setter must surely have been seeking an award for the most ambiguous problem wording! Any one of those four is a reasonable interpretation of the actual wording!
  6. Sep 19, 2012 #5


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    I have already tried 1 and 4 and they did not work. Give me some hints for 2 and 3.
  7. Sep 19, 2012 #6
    OK I have had another look at the detail of your work.
    Your answer is quite correct. The other three possible answers according to my last posting would be (1) 0.43 (2) 0.57 or (3) 1.30. The 2.30 you have quoted is the correct value for option (4).

    To understand how to arrive at (2) and (3) simply separate each of the two terms in the fraction in my formulae for these interpretations.
  8. Sep 20, 2012 #7


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    OK the correct answer is 1.0565. But none of the interpretations match with the answer. Only the 3rd interpretation is somewhat close to the answer. So should I go with the third interpretation of yours?
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