Nuclear Decay HalfLife Practice Radioactive HalfLife Practice Problems

Nuclear Decay Half-Life Practice

Radioactive Half-Life Practice Problems 1. How many grams of iodine 131 (half life- 5 days) would be left after 20 days if you start with 25 grams? Answer: 1. 56 g 5 days The half life is Number of half-lives passed Amount of Matter Time 0 { days} 0 25 g 1 12. 5 g 5 days X 1/2 2 6. 25 g 10 days X 1/2 3 3. 12 g 15 days 4 1. 56 g 20 days 1/2 X 1/2 1/16 amount left over 5

2. How long will it take 600 grams of Plutonium 239 (half life 24, 000 years) to decay to 18. 75 grams? 120, 000 yrs 24, 000 yrs The half life is Number of half-lives passed Amount of Matter 0 600 g 0 yrs 1 300 g yrs 24, 000 2 g 150 48, 000 yrs 3 g 75 72, 000 yrs 4 37. 5 g 5 18. 75 g Time yrs 96, 000 120, 000 yrs

3. K-42 has a half-life of 15. 5 hrs. If 13. 125 g of K-42 remains undecayed after 62. 0 hours, what was the original sample size? 210 g 15. 5 hrs The half life is Number of half-lives passed Amount of Matter 0 210 g 1 105 g 2 52. 5 g 31 hrs 3 26. 25 g 46. 5 hrs 4 13. 125 g 62 hrs Time 0 hrs 15. 5 5

4. An isotope of cesium (cesium-137 has a half -life of 30 years. If 20 mg of cesium-137 disintegrates over a period of 90 years, how many mg of cesium-137 would remain? 2. 5 mg 30 years The half life is Number of half-lives passed Amount of Matter Time 0 20 mg 0 Years 1 10 mg 2 5 mg 60 Years 3 2. 5 mg 90 Years 4 30 Years 5

5. Thallium-208 has a half-life of 3 min. How long will it take for 120. 0 g to decay to 7. 50 g? 3 minutes The half life is Number of half-lives passed Amount of Matter Time 0 120 g 0 minutes 1 60 g 3 minutes 2 30 g 6 minutes 3 15 g 9 minutes 4 7. 5 g 12 minutes 5 12 minutes

6. If 60 g of Lithium-9 has a half-life of 100 years, how long will it take for lithium-9 to decay to 15 g? 100 years The half life is Number of half-lives passed Amount of Matter Time 0 60 g 100 years 1 30 g 200 years 15 g 2 300 years 3 4 5 300 years