Half-Life of Radioactive Isotopes
1. How much of a 100.0 g sample of 198Au is left after 8.10 days if its half-life is 2.70 days?
2. A 50.0 g sample of 16N decays to 12.5 g in 14.4 seconds. What is its half-life?
3. The half-life of 42K is 12.4 hours. How much of a 750 g sample is left after 62.0 hours?
4. What is the half-life of 99Tc if a 500.0 g sample decays to 62.5 g in 639,000 years?
5. The half-life of 232Th is 1.4 x 1010 years. If there are 25.0 g of the sample left after 2.8 x 1010 years,
how many grams were in the original sample?
6. There are 5.0 g of 131I left after 44.85 days. How many grams were in the original sample it ifs halflife is 8.97 days?
7. Cobalt-60 is used in cancer radiation therapy. If you start with 4.516 x 108 atoms of Cobalt-60, how
much time will pass before the amount is reduced to 1.764 x 106 atoms? The half-life is 5.26 years.
8. The half-life of 71Zn is 2.40 minutes. How long will it take for 32.0 g of zinc-71 to decay to 2.00
grams?
9. A radioactive-tracer, sodium-24, is used to study circulatory problems. If you start with 5.85 x 1023
atoms, how many atoms will remain after 12.0 days? The half-life of 24Na is 4.0 days.
10. Some patients with thyroid disorders receive radioactive iodine-131. The half-life 131I is 8.07 days. If
you start with 7.73 x 1012 atoms, how much time will pass before the amount is reduced to 2.42 x 1011
atoms?
11. Batteries used in heart pacemakers contain plutonium-238. If your original sample contains 2.57 x 109
atoms of 238Pu, how much time will pass before the amount is reduced to 5.02 x 106 atoms? The halflife is 27.1 years.
12. Carbon-14 has a half-life of 5730 years and is used for radioactive dating. Assume we started with
2.18 x 1017 atoms of 14C in a wood sample obtained from a wooden beam from an ancient tomb.
There are 1.09 x 1017 atoms remaining. How old can we assume the wooden beam to be?