If a radioactive isotope has a half-life of 100 years. A sample of 40 grams of the isotope will have a mass of how many grams in 200 years?
Half-life (t½) is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period.
In this question (t½) of isotope is 100 years, which means that after 100 years half of the sample would have decayed and half would be left as it is.
After 100 years ( first half life) 40 /2 = 20 g decays and 20 g remains left.
After another 100 years ( two half lives or 200 years) 20 /2 = 10 g decays and 10 g remains left .
after two half lives or 200 years, 10 g of the isotope will be left.