1. The problem statement, all variables and given/known data One of the many isotopes used in cancer treatment is 19879Au, with a half-life of 2.70 d. Determine the mass of this isotope that is required to give an activity of 200 Ci. 2. Relevant equations 200Ci x 3.7x10^10 decays/s = 7.4 x 10^12 decays/s T1/2 = ln2/lambda = 0.693/lambda 3. The attempt at a solution I looked through my book in the section on half-life and radioactive dating, but could not find an equation(s) that would seem useful for this problem, since it involves mass (which is introduced later on in the chapter). The only equation I found relevant is the one I listed in (b). Any help would be appreciated. Thanks!