Radioactivity Half-life Problem

Question

If an original sample of ^91 Mo contains 2.97 x 10^22 atoms, how many atoms will remain after 62 minutes? The half life of ^91 Mo is 15.5 minutes.

Answer 1

The easiest way to do these problems is to use the formula

F = 0.5ⁿ

F = fraction remaining

n = number of half lives that have elapsed

n = 62 minutes x 1 half-life/15.5 minutes = 4 half lives have elapsed

F = 0.5ⁿ

F = 0.0625 left

Atoms left = 2.97x10²² atoms x 0.0625 = 1.86x10²¹ atoms remain