Half-Life Practice Problems


1.) What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 grams in 21.6 seconds?


2.) All isotopes of technetium are radioactive, but they have widely varying half-lives. If an 800.0 gram sample of

technetium-99 decays to 100.0 g of technetium-99 in 639,000 years, what is its half-life?

3.) A 208 g sample of sodium-24 decays to 13.0 g of sodium-24 within 60.0 hours. What is the half-life of this
radioactive isotope?


4.) If the half-life of iodine-131 is 8.10 days, how long will it take a 50.00 g sample to decay to 6.25 g?


5.) The half-life of hafnium-156 is 0.025 seconds. How long will it take a 560 g sample to decay to one-fourth
of its original mass?


6.) Chromium-48 has a short half-life of 21.6 hours. How long will it take 360.00 g of chromium-48 to decay
to 11.25 g?


7.) Potassium-42 has a half-life of 12.4 hours. How much of an 848 g sample of potassium-42 will be left after
62.0 hours?


8.) Carbon-14 has a half-life of 5730 years. How much of a 144 g sample of carbon-14 will remain after
1.719 104 years?


9.) If the half-life of uranium-235 is 7.04 108 years and 12.5 g of uranium-235 remain after 2.82 109 years,
how much of the radioactive isotope was in the original sample?


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