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Initially, a particular sample has a total mass of 240 grams and contains 512 x 1010 radioactive nuclei. These radioactive nuclei have a half life of 1 hour. (a) After 4 hours, how many of these radioactive nuclei remain in the sample (that is, how many have not yet experienced a radioactive decay)? Note that you can do this problem without a calculator. 32 x 1010 radioactive nuclei (b) After that same amount of time has elapsed, what is the total mass of the sample, to the nearest gram? 15 X g After exactly 5 hours, the activity level of a sample of a particular radioactive isotope, which decays into a stable isotope, has fallen to 30.0% of its initial value. Calculate the half-life of this isotope. L