Question

Calcium-49, a radioactive isotope, has a half-life of 9 minutes. If an 80-gram sample of the isotope decays for 27 minutes, how many grams of the original sample remain?

Select one:
a. 60
b. 10
c. 40
d. 20​

Answer 1

Half-Life is the time required for half of the radioactive nuclei to decay to products.

In this case Calcium-49 has a half-life of 9 minutes and is allowed to decay for 27 minutes, meaning the sample experienced 3 half-lives.

80g of Calcium-49 allowed to decay for 9 minutes results in 40g of Calcium-49 (half-life 1)

40g of Calcium-49 allowed to decay for 9 minutes results in 20g of Calcium-49 (half-life 2)

20g of Calcium-49 allowed to decay for 9 minutes results in 10g of Calcium-49 (half-life 3)

b) 10g of Calcium-49 remains after 27 minutes of decay

hope this helps :)