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Homework Help: Uranium Dating

  1. Jan 4, 2015 #1
    1. The problem statement, all variables and given/known data
    The isostopc abundances of a sample is U-235 and U-238 are 0.72 and 99.27 respectively; what is the age of the sample? (assume isotope abundance was equal when sample was formed)

    2. Relevant equations
    [itex]\lambda=\frac{ln2}{ t_{\frac{1}{2}}}[/itex]

    3. The attempt at a solution
    for U-238 [itex]N_{238}(T)=N_{238}(t)e^{\lambda _{238}t}[/itex]
    U-235 [itex]N_{235}(T)=N_{235}(t)e^{\lambda _{235}t}[/itex]
    T is time at present and t is time of sample formation.

    diving the two equations gives
    [itex] \frac{N_{238}(T)}{N_{235}(T)}=\frac{N_{238}(t)}{N_{235}(t)}e^{(\lambda _{238}- \lambda _{235})t}[/itex]

    From the assumption, one can say [itex]\frac{N_{238}(t)}{N_{235}(t)}=1[/itex]
    It's here where i'm not sure; do i just say that [itex]\frac{N_{238}(T)}{N_{235}(T)}=\frac{99.27}{0.72}[/itex] and solve for t or am I missing something? Thanks

    EDIT: Sorry, I can't get my latex to work, I can't seem to fix it
    Last edited: Jan 4, 2015
  2. jcsd
  3. Jan 4, 2015 #2


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    2017 Award

    Staff: Mentor

    I fixed the two broken equations, but the first one looks odd.
    Yes, just do that and solve for t.
  4. Jan 4, 2015 #3
    Thanks for that, I fixed the first equation.
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