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Homework Help: Calculate the age of the sample.

  1. Nov 2, 2012 #1

    I've been given this question as my homework over the holiday, and I've come to attempt it and I'm completely stumped, as are most in my class so could you guys point me in the right direction?


    Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.

    my guess..

    Just thinking over as I write it, would I have to see how long it would take the uranium to decay completely then divide it by 7?
    Last edited: Nov 2, 2012
  2. jcsd
  3. Nov 2, 2012 #2
    Think about what half-life means.
    after 4,000,000,000 years how much is left?
    after 8,000,000,000 years?
    after 12,000,000,000 years?
    after 16,000,000,000 years?
    Last edited: Nov 2, 2012
  4. Nov 2, 2012 #3
    Right so,

    After 4b years 119 is left.
    After 8b years 60 is left.
    After 12b years 30 is left.
    After 16b years 15 is left.

    Then what would I do?
  5. Nov 2, 2012 #4
    Where do you get 119 from?:bugeye:
  6. Nov 2, 2012 #5
    Uranium 238 / 2 = Uranium 119?:S
  7. Nov 2, 2012 #6
    Starting out with a sample of uranium 1/2 of it will be lead after 4b years the other half will still be uranium. How much lead and how much uranium will there then be after another 4b years have gone by (man this stuff decays slowly!)?
  8. Nov 2, 2012 #7
    It will be 1/4 uranium to 3/4 led..

    So do I keep going till it is 1/8 uranium to 7/8 lead?
    Last edited: Nov 2, 2012
  9. Nov 2, 2012 #8
    Nah! - that's not what half-life means - Basic_Physics just posted correctly
  10. Nov 2, 2012 #9
    Now you got it.
  11. Nov 2, 2012 #10
    So it's not that hard, thanks:):)
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