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Half life practice problems



                                             Half Life Questions

   1. A new and very rare radioactive element, Taylorium, was found encased in a large piece of ice in
      Stamford, CT. Stamford chemists have been working night and day to retrieve samples of Taylorium
      and conduct experiments to determine more about Taylorium’s radioactivity.
         a. What is Taylorium’s half life if a 100 gram sample degrades to 25 g in just 30 minutes?

           b. How much Taylorium would be left if we started with 86,260 g and 4 half lives have gone by?

           c. If we currently have 10,782.5 g of Taylorium, and we started with 86,260 g, how much time
              has elapsed?

           d. What fraction of Taylorium will be left after 90 minutes have passed?

   2.   How much of a 100.0 g sample of gold-198 is left after 8.10 days, if its half life is 2.70 days?

   3. A 50.0 g sample of nitrogen-16 decays to 12.5 g in 14.4 seconds. What is its half life?

   4. The half life of potassium-42 is 12.4 hours. How much of a 750. g sample is left after 62.0 hours?

   5.   What is the half life of 99Tc if a 500.0 g sample decays to 62.50 g in 639,000 years?

   6. The half life of 232Th is 1.4x1010 years. If there are 25.0 g of the sample left after 2.8x1010 years,
      how many grams were in the original sample?
   7. There are 5.0 g of iodine-131 left after 40.35 days. How many grams were in the original sample if
      its half life is 8.07 days?

   8. Fluorine-21 has a half life of approximately 5 seconds. What fractio of the original amount will be
      present after 1 minute? If you start with 21 grams of fluorine, how many grams will remain after 1

   9. The half life of chromium-51 is 28 days. If a sample initially contains 510 grams, how much
      chromium will remain after 56 days? How much will remain after 224 days?

   10. If you have a 4.0 g sample of fluorine-21 with a half life of approximately 5 seconds, how long will it
       take to have only 1.0 g remaining?

   11. If, after 175 days, you have a sample of 12.2 g of chromium-51 (half life of 25 days), how much
      chromium did you start out with?

   12. A new radioactive element, Martelium, has been synthesized. What is the half life of Martelium if a
       50.0 g sample decays to 25.0 g in 12 days?

For additional practice:
Relevant section in book is all of Chapter 19
Additional practice problems can be found on p.629-631 (3, 4, 6-9, 12-15, 21, 26, 34, 36, 37, 44, 48, and 55)

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