The half-life of phosphorus-30 is 2.5 minutes. If you start with 35g oh phosphorus-30, how many grams would remain after 20.0 minutes?

0.137 g The half-life of a substance is the time it takes for half of the substance to decay due to spontaneous radioactivity. Doesn't matter how much you start with, after 1 half-life, you will have half of what you started with. So you need to determine how many half lives have gone by. You get this by dividing the total time period by the half-life p = 20.0 min / 2.5 min = 8 So you have 8 half lives. To get the amount of material left, you divide by 2^p where p is the number of half-life periods spent. So 35 g / 2^8 = 35 g / 256 = 0.136719 g. Rounded to 3 significant digits and you have only 0.137 grams of phosphorus-30 after having started with 35g, 20.0 minutes earlier.