University:
College
Course:
College Physics
Academic year:

2022
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498

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2
Author:

Kwadiso3o

Name: _____________ANSWER KEY________________ Date: ______________________________ Period: _______ Radioactivity and Half-Lives Review Worksheet 1. What is a half-life? The time it takes for half of the material you started with to decay. 2. If we start with 400 atoms of a radioactive substance, how many would remain after one halflife? 200 After 2 half-lives? 100 After 3 half-lives? 50 After 4 half-lives? 25 3. If we start with 16 grams of a substance, how much will remain after 3 half-lives? 16  8  4  2 2 grams after 3 half-lives. 4. A sample of carbon-14 has been decaying for 22,920 years and is now 35.0 grams. What was the size of the original sample? (The half-life of carbon-14 is 5,730 years) 22920/5730 = 4 half lives You will need to work backwards 35  70  140  280  560. grams 7. How old is a skeleton sample if the current amount of carbon in the bones is 3.125%? (Assume you started with 100%) 100%  50  25  12.5  6.25  3.125 5 half lives x 5,730 = 28,650 years 8. If you started with 120 grams of a radioactive substance, and now you have 15 grams, how many half-lives have passed? 120  60  30  15 3 half-lives 9. The half-life of isotope X is 2.0 years. How many years would it take for a 4.0 mg sample of isotope X to decay and have only 0.50 mg of it remain? 4.0  2.0  1.0  0.50 3 half lives x 2 years = 6.0 years 10. The half-life of Po-218 is three minutes. How much of a 200 atom sample will remain after 15 minutes? 15 minutes/3 = 5 half lives 200  100  50  25  12.5  6.25 atoms 11. The half-life of Au-198 is 2.69 days. How long does it take a 180g sample to decay to 1/8 its original mass? 1  ½  ¼  1/8 3 half lives x 2.69 days = 8.07 days 12. What is the half-life of a radioactive sample if 100.0 grams of it decays to 12.5 grams in 24.3 hours? 100  50  25  12.5 24.3/3 half lives = 8.10 hours 13. How long will it take a sample of 131I to decay to 12.5% assuming its half-life is 8.07 days? 100%  50  25  12.5 3 half lives x 8.07 = 24.2 days 14. Sodium-25 was to be used in an experiment, but it took 3.0 minutes to get the sodium from the reactor to the laboratory. If 5.0 mg of sodium-25 was removed from the reactor, how many mg of sodium-25 were placed in the reaction vessel 3.0 minutes later if the half-life of sodium-25 is 60 seconds? 3 minutes / 60 seconds = 3 half lives 52.5  1.25  0.625mg 15. Actinium-226 have a half-life of 29 hours. If 10.0 g of actinium-226 disintegrates over a period of 145 hours, how many mg of actinium-226 will remain? **You need to do dimensional analysis to solve this one! ** 145 / 29 = 5 half lives 1.25  0.625  0.3125g 0.3125 g x 103 mg = 312.5 mg 1g 10.0g  5.0  2.5 