This document contains 6 problems about calculating the amount of radioactive material remaining after various half-lives. The problems involve calculating the remaining mass of several isotopes, including gallium-68, rutherfordium-261, strontium-90, iron-59, arsenic-73, and thallium-206, given their original masses and half-lives in days, years, minutes, or seconds.

1. Half-Life Problems
1. How much of a 10.0 gram sample of gallium-68 will be left after 3 complete half-lives?
2. Element 104, Rutherfordium, has a radioactive isotope with a half-life of 65 seconds. If you start
with 500 grams of Rf-261, how much will be left after 130 seconds? After 260 seconds? After 650
seconds?
3. Sr-90 has a half-life of 29 years. If there is 100 grams present in a rock, how many grams of Sr-90
will remain after 87 years?
4. Iron-59 is used in medicine to diagnose blood circulation disorders. The half-life of iron-59 is 44.5
days. How much of a 20.0 g sample will remain after 133.5 days?
5. The half-life of As-73 is 80 days. If you begin with 100 grams,
a. how long will it take until only 50 grams remain?
b. how long until only 12.5 grams is left?
6. A rock contains 200.0 g of Thallium-206. The half-life of Tl-206 is 4.77 minutes. How many grams
of Tl-206 will remain after 14.3 minutes have passed?