We’re being asked to calculate the amount of iodine (I) remaining after 3 half-lives if we start with 1 mg sample.
Recall that half-life is the time needed for the amount of a reactant to decrease by 50% or one-half. One way to determine the amount remaining after x half-lives is:
where [A]t = concentration or amount after x half-lives
[A]0 = initial concentration or amount
and x = number of half-lives.
Americium-241 is used in smoke detectors. It has a first order rate constant for radioactive decay of k = 1.6 x 10-3 yr-1. By contrast, iodine-125, which is used to test for thyroid functioning, has a rate constant for radioactive decay of k = 0.011 day-1.
How much of a 1.00 mg sample of iodine remains after 3 half-lives?