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Homework Help: Age of rocks calculation - I only need little help

  1. Apr 19, 2012 #1
    How many half lives have passed since a particular rock was formed if a sample of the rock contains 31 times as much lead-207 than uranium-235?

    I know that I need to do a table and calculate

    I know how to do this question but the thing I need help on is how do I know the number of atoms in order to do a table and calculate. Like I have the ratio of parents:daughter isotope which is 1:31 right? but how do I do this? Just tell me how and I will do it myself. Actually I don't know physics, I'm studying it all by myself and I got this question from my friend's homework and I'm stuck on it, there are 3 questions like it and I need help on how to turn this ratio into actual numbers in order to calculate how many half lives have passed. If I don't need to turn it into numbers then please tell me how do I solve this question? Thank you :)

    EDIT: okay so thats what I did is this correct?

    1 - 31 half lives=5
    2 - 30 half lives=4
    4 - 28 half lives=3
    8 - 24 half lives=2
    16 - 16 half lives=1
    32 - 0 half lives=0
    so 5 half lives have passed
    Last edited: Apr 19, 2012
  2. jcsd
  3. Apr 19, 2012 #2


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    5 half lives is correct.

    Last edited: Apr 19, 2012
  4. Apr 19, 2012 #3
    Hmm? You didn't really explain anything...how did you get to 32? I got to 5.
  5. Apr 19, 2012 #4


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    I meant 5, sorry... As the original amount decreased to 1/32-th.

  6. Apr 19, 2012 #5
    oo thank you :)..and my way is correct do I have to do any formulas?
    Thank you very much ^^
  7. Apr 19, 2012 #6


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    The daughter atoms present + number of un-decayed U235 atoms=N(0):
    31N + N =N(0)

    The number of un-decayed isotopes present at time t is

    N(t)=N(0) (1/2)t/halflife,

    where N(0) is the number initially.

    N=N(0)/32, 32 =25, ....

  8. Apr 19, 2012 #7
    I see :D Now I understand everything, thank you!
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