Question

You have 100 grams of a substance that has a half life of 10 years. How much of the original substance will you have after 20 years have passed?

Answer 1

Answer:

So here goes. Half-life means that after that many days, half of the sample is left. So if you start with 2 grams of a substance that has a half-life of 10 days, then

After 10 days, you will have 1 gram.

After 20 days, you will have 1/2 gram

After 30 days, you will have 1/4 gram

After 40 days, you will have 1/8 gram

Note that the amount is cut in half each ten days. A common mistake made in chemistry classes is when the student says, “Okay, half of it is gone in the first 10 days and the other half is gone in the second ten days, and then you have no more.”

It doesn’t work that way. It halves your current amount each 10 days. Technically, you never actually get to zero, but after enough time the amount that keeps getting halved is so tiny it might as well be zero.