# Honors Chem Essay example

1223 Words 5 Pages
Directions: The following questions involve more practice with radioactive decay half-life. Complete the problems to the best of your ability. This assignment is due by next WEDNESDAY, November 16th.

1. If 100.0 g of carbon-14 decays until only 25.0 g of carbon is left after 11 460 y, what is the half-life of carbon-14? a. Calculate how many half-lives have passed during the decay of the 100.0 g sample.
100 grams/2 50 grams/2 25grams…2 half lives have passed

b. Solve for the half-life.
11,460year/2 half lives = 5,730 years half-life of carbon – 14

2. What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 g of nitrogen-16 in 21.6 s? 100g50g25g12.5g…3 half lives have passed 21.6s/3 half
Chromium-48 has a short half-life of 21.6 h. How long will it take 360.00 g of chromium-48 to decay to 11.25 g? 360g/2 180g/2 90g/2 45g/2 22.5g/2 11.25g…5 half lives have passed 21.6 hrs x 5 half lives = 108 hrs

11. Gold-198 has a half-life of 2.7 days. How much of a 96 g sample of gold-198 will be left after 8.1 days? a. Calculate how many half-lives have passed during the decay of the 96 g sample.
8.1days/2.7days = 3 half lives pass

b. Calculate how much of the sample will remain after 3.0 half-lives. 96g/2 48g/2 24g of gold – 198 remain after 3 half lives

12. Potassium-46 has a half-life of 12.4 hours. How much of an 848 g sample of potassium-46 will be left after 62.0 hours? 62hrs/12.4hrs = 5 half lives pass 848g/2 424g/2 212g/2 106g/2 53g/2 26.5g of potassium – 46 remain after 5 half lives
13. Carbon-14 has a half-life of 5730 yrs. How much of a 144 g sample of carbon-14 will remain after 17190yrs? 17,190yrs/5,730yrs = 3 half lives pass 144g/2 72g/2 36g/2 18g of carbon – 14 remain after 3 half lives

14. If the half-life of uranium-235 is 704000000 yrs and 12.5 g of uranium-235 remain after
2820000000 yrs, how much of the radioactive isotope was in the original sample? 2820000000yrs/704000000yrs = 4 half lives have passed WORKING BACKWARDS in time… 12.5g x 2 25g x 2 50g x 2 100g x 2 200g = the original amount of uranium –

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