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11 December, 18:56

Cobalt-63 has a half-life of 5.3 years. If a pellet that has been in storage for 15.9 years contains 40.0g of Cobalt-63, how much of this radioisotope was present when the pellet was put into storage? Show work.

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  1. 11 December, 21:16
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    320 g

    Step-by-step explanation:

    The half-life of Co-63 (5.3 yr) is the time it takes for half of it to decay.

    After one half-life, half (50 %) of the original amount will remain.

    After a second half-life, half of that amount (25 %) will remain, and so on.

    We can construct a table as follows:

    No. of Fraction Mass

    half-lives t/yr Remaining Remaining/g

    0 0 1

    1 5.3 ½

    2 10.6 ¼

    3 15.9 ⅛ 40.0

    4 21.2 ¹/₁₆

    We see that 40.0 g remain after three half-lives.

    This is one-eighth of the original mass.

    The mass of the original sample was 8 * 40 g = 320 g
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