Iodine-131 has a half life of eight days .If you start with 32g of I-131 how much will you have after 8 days!!?

Half life is simply the time it takes for one half of a sample to disappear. If you start out with 32 grams of I-131, and it has a half life of eight days, half of that 32 grams will be gone after 8 days. One half of 32 is 16, leaving you with 16 grams remaining.