Iodine-131, a beta emitter, has a half-life of 8 days. a 8-gram sample of initially pure iodine-131 is stored for 32 days. how much iodine-131 remains in the sample afterward?

The answer is a 1/2 gram or 0.5 grams. After 8 days, there will be 4 grams. After 16 days, there will be 2 grams. After 24 days, there will be 1 gram. And after 32 days, there will be a 1/2 gram or 0.5 grams. The formula is

(1/2) ^n = fraction remaining

where n is the number of half-lives.

In this case, n=4, since 32/8=4.

The formula gives us

(1/2) ^4=1/16

1/16 of 8 grams (the starting amount) is 1/2 gram or 0.5 grams