KarenNC Posted May 9, 2015 Share Posted May 9, 2015 Here's the question (from biology): 3. UraniumÂ238 decays into lead. For a given amount of 238U, half is converted to lead every 4.5 million years. a) If a sample is found with 1/8 (0.125) of the expected 238U, how old would this sample be? (2 points) Now, using A for the beginning amount and with my current understanding of the process, it would work out as follows: A begin count 1/2 A 4.5 million years (one half-life) 1/4 A 9 million years (two half-lives) 1/8 A 13.5 million years (three half-lives) So, three half-lives, so the sample would be about 13.5 million years old. Other examples online I've looked at for similar problems seem to follow this pattern. The answer given in the key, however, says that it would be three half-lives, but about 18 million years old. Is the answer key in error, or have I missed an important step? Quote Link to comment Share on other sites More sharing options...
Arcadia Posted May 9, 2015 Share Posted May 9, 2015 13.5 million years is correct The same radioactive decay is taught in physics. I actually calculated using the generic natural log radioactive formula for fun. You can input at below link and get the same answer. You just need to choose Uranium 238 https://www.ehs.washington.edu/rso/calculator/activity_calc.shtm ETA: Example of a typical radioactive decay graph http://www.launc.tased.edu.au/online/sciences/PhysSci/done/nuclear/decay/Decay.htm ETA: Weird thing is U238 is listed as having a half life of 4.5 billion years at some sites like Duke and USGS http://pubs.usgs.gov/gip/geotime/radiometric.html http://people.chem.duke.edu/~jds/cruise_chem/nuclear/chemwindow/halflife.html 1 Quote Link to comment Share on other sites More sharing options...
KarenNC Posted May 9, 2015 Author Share Posted May 9, 2015 Thanks! Quote Link to comment Share on other sites More sharing options...
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