Homework Help: ChemB

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

The activity of carbon 14 in living tissue is 15.3 disintegrations per minute per gram of carbon. The limit for reliable determination of age is based on carbon 14 is .10 disintegration per min per gram of carbon. Calculate the max age of a sample that can be dated accurately by radiocarbon dating. The half life is 5730 years.

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 1:05am
I 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 1:15am
Oh, 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 1:16am
How do you know what number goes on the top and bottom for the disintegration?
ChemB - DrBob222, Monday, January 23, 2012 at 12:49pm
No. 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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