Chem 2

The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration.

a) How long will it take for 10% of the U-238 atoms in a sample of U- 238 to decay?

b)If a sample of U-238 initially contained 1.9×1018 atoms when the universe was formed 13.8 billion years ago, how many U-238 atoms will it contain today?

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  1. k = 0.693/t1/2
    Then
    ln(No/N) = kt
    I would call No = 100
    Then N = 90(after 10 have decayed--which is 10%)
    k from above. Solve for t in years.

    b.
    Same procedure. No = atoms you started with. N = atoms remaining.

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  2. hb

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  3. 100

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