The half-life of As-81 is 33 seconds. If only 12.5 g of a 100.0 g sample of As-81 is left, how long did it take for the original sample to decay to its current mass?

Question

Quizplus · Accepted Answer

We first need to determine how many half-lives have occurred to create a mass of 12.5 g from the original 100 g sample.
100 → 50 → 25 → 12.5
This means that three half-lives have occurred. Since each half-life is 33 seconds, multiply 33 by 3 to obtain 99 seconds as the time it has taken for the As-81 to decay.

The Half-Life of As-81 Is 33 Seconds