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The half-life of iodine -125 is 60 days. What fraction of iodine-125 would be left after 180 days?

my teacher didn't teach us this and expects us to know it and I don't understand!!!
could you explain it please!?

  • Chemistry -

    The half life of a material is the time it takes for exactly 1/2 of the material you started with to disintegrate into another material. In this case, the half life is 60 days. Therefore, 1/2 will be left after 60 days, 1/2 of the 1/2 (that is 1/4) will be left after 120 days (a second half life) and 1/2 of the 1/4 (=1/8) will be left after 120 days (the third half life).
    The amount left is 1/2n where n is the number of half lives.
    1/2n = 1/23 = 1/8

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