1.12% of a certain radioisotope decay in 5.2 years, what is the half-life of this isotope?
2. Charcoal found under a stone at Stonehenge, England, has a carbon-14 activity that is 0.60 that of new wood. How old is the charcoal? (The half-life of carbon-14 is 5,730 years.)
3. The radioisotope potassium-40 decays to argon-40 by positron emission with a half-life of 1.3 × 109 yr. A sample of moon rock was found to contain 78 argon-40 atoms for every 22 potassium-40 atoms. The age of the rock is
4. The isotope of iodine with mass number 128 undergoes beta decay with a half-life of 25.00 minutes. How long will it take for 93.75% of a sample of pure 128 I to decay?
5. Petroleum is a fossil fuel containing many different carbon compounds. If the carbon atoms in petroleum have been in the ground for 100 million years, what fraction of the initial 14C atoms is still there? (t1/2 = 5,370 yr)
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