Chemistry 111 Half-Life Problems Chapter 2
1. If you start with 5.32 x 109 atoms of 129 Cs, how much time will pass before the
amount is reduced to 5.20 x 106 atoms?
2. If you start with 5.80 x 10 28 atoms of 242 Pu, how many atoms will remain in
3.03 x 106 years?
3. A radioactive sample has a half-life of 12 d. What % of the sample remains
after a) 24d b) 26d c) 55d ?
4. If 30% of a sample is left after 7 years, find its half-life.
5. When will 10% of a sample remain if its half-life is 1892 years?
6. If there are 9.6 g of 242 Pu remaining from an original amount of 115.2 g, how
much time will have gone by?
7. Some patients with thyroid disorders receive radioactive iodine-131. The halflife of 131 I is 8.07 days. If you start with 7.73 x 10 12 atoms how much time will
pass before the amount is reduced to 7.55 x 109 atoms?
8. If 22% of a sample is left after 121 years, find its half-life.
ANS. 1. 321 h
2. 2.27 x 1026
3. 25% 22% 4%
4. 4.03 years
atoms
5. 6285 years
6. 1.4 x 10 6 years 7. 80.7 d
8. 55.5 years