1. The problem statement, all variables and given/known data Archeologists removed some charcoal from a Native American campfire, burned it in O2, and bubbled the CO2 formed into Ca(OH)2 solution (limewater). The CaCO3 that precipitated was filtered and dried. If 4.58 g of the CaCO3 had a radioactivity of 3.2 d/min, how long ago was the campfire? 2. Relevant equations C + O2 --> CO2 CO2 + Ca(OH)2 --> CaCO3 + H2O 3. The attempt at a solution Activity = kN So k = Activity/N = (3.2 atoms/min)/(0.04576 mol * 6.022*10^23 atoms/mol) = 1.2*10^-22 min^-1 half life = ln2/k = 5.776*10^21 min And that's all I know..I don't have the initial and final moles of the CaCO3, so how should I calculate the time that passed?