Iodine-131 has a half life of 8.040 days how long will it take for a 40.0 grams sample of iodine 131 to decay to 0.75g

Explanation:

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In this case, since the radioactive decay is considered as first-order kinetic model, whereby the remaining mass of the radioactive material involves the initial one, the rate constant and elapsed time:

As we were not initially given the rate constant, we can use the half-life to calculate it as follows:

Thus, we can calculated the elapsed time for the given conditions to obtain:

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