How much of radioactive substance remains after 15 hours if it's half life is 5 hours?

Answer 1

The half-life of a radioactive substance is the time that it takes for half of the atoms to decay. With a half-life of 10 days, half has decayed in this time. After 20 days, a further 10 days/another half life, a further half of the remainder has decayed, so 1/4 of the original material remains, 1/4 of 15g is 3.75 grams. This is the amount of original radioactive substance remaining, but it’s daughter isotope ( what the decay has produced ) is also present, so the original sample mass is effectively constant, especially in a sealed container. Even in an unsealed container, and assuming alpha ( helium nucleii) emission, a drop in mass per radioactive atom of 4 Atomic Mass units, compared with the original atom of, say 200 amu is only 2% mass decrease, less for heavier decaying nucleii.