25. Iodine-131 is a radioactive substance that can be used to treat certain thyroid conditions. It has a

balf-life of 8 days. Your friend with a thyroid condition is given 40 mg of I-131 to drink. Assuming

that all the l-131 stays in her body, how much will remain after 4 half-lives?

2.5 mg of I-131

Explanation: Half life refers to the time taken by a radioactive isotope to decay to a half of its initial amount. For example, if the half-life of I-131 is 8 days, it will take 8 days for a mass of 100 g of I-131 to decay to 50 g.

In this case, we are given;

Initial mass of I-131 is 40 mg

Half life of I-131 is 8 days

Number of half lives is 4.

We are required to determine the mass that will remain after 4 half lives.

Using the formula;

Remaining mass after decay = Initial mass before decay * (1/2) ^n, where n is the number of half lives.

Therefore;

Remaining mass = 40 mg * (1/2) ^4

= 40 mg * 1/16

= 2.5 mg

Hence, the mass I-131 remaining in her body after 4 half-lives will be 2.5 mg.