Question

The half-life of a radioactive isotope is the time it takes for a quantity of the isotope to be reduced to half its initial mass. Starting with 220 grams of a radioactive isotope, how much will be left after 3 half-lives?

Answer 1

Quantity remaining= 27.5 grams

Explanation:

If the initial quantity of the radioactive isotope= 220 grams

And for each half life it will reduce to half of its original quantity

For first half life

Quantity= 220/2

Quantity= 110 grams

Now for the second half life , the quantity to work with will be 110 grams

Second half life

Quantity= 110/2

Quantity= 55 grams

For the third half life , the quantity to work with is 55 grams

Third half life

Quantity= 55/2

Quantity= 27.5 grams

So after 3 half life's of this radioactive isotope, the amount remaining out of 220 = 27.5 grams.

Another simple way to do this

Since half life reduce the isotope by half of its original.

The third half life will reduce it by (1/2)^3= 1/8

So the quantity remaining= 1/8*220

Quantity remaining= 27.5 grams