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October 20, 2014

October 20, 2014

Posted by **L.Bianchessi** on Monday, January 23, 2012 at 12:49am.

If the half life is 5730 I get 0.123 years for k. The constant. How do I use the other information to get the max age of the sample?

- ChemB -
**DrBob222**, Monday, January 23, 2012 at 1:05amI don't agree with your value for k.

k = 0.693/t_{1/2}= 0.0001209 Yr^-1

If we can detect 0.1 dpm, then

ln(15.3/0.1) = kt and solve for t. The web states that 58,000 to 62,000 years can be reliable determined by rediocarbon dating. This is in that ball park.

- ChemB -
**L.Bianchessi**, Monday, January 23, 2012 at 1:15amOh, okay. Do I always use that equation for any order rxn involving time? How about the units, would it be yrs^-1 or just years?

- ChemB -
**L.Bianchessi**, Monday, January 23, 2012 at 1:16amHow do you know what number goes on the top and bottom for the disintegration?

- ChemB -
**DrBob222**, Monday, January 23, 2012 at 12:49pmNo. ln(No/N) = kt is good only for first order AND in it's complete form is ln(No/N) = akt.

No = number of atoms initially

N = number of atoms at time t.

Note: usually number; however, dpm, concn, or other initial/final states work as well. In this case I used 15.3 for No since that is the original dpm (disintegrations/min) and 0.1 dpm since that is the smallest the problem states that can be reliably determined.

If t_{1/2}is in years, then k is in years^-1 and t is in years. If half life is in seconds, then k is in seconds^-1 and t is in seconds.

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