Answer :
According to the concept of half-life 125 g of strontium-90 would be left after 116 years.
What is half-life?
Half -life of a substance is defined as the time which is required for half of the quantity of a radioactive substance to get decayed.It is a term which is used in nuclear chemistry for describing how quickly unstable atoms undergo radioactive decay into other nuclear species by emitting particles or the time which is required for number of disintegrations per second of radioactive material to decrease by one half of its initial value.
To calculate mass of strontium-90 after 116 years number of half-lives is calculated during 116 years as, 116/20= 4 half lives
During 1st half life, mass of Sr-90= 2000/2=1000 g
During 2nd half life,mass of Sr-90=1000/2=500 g
During 3 rd half-life,mass of Sr-90=500/2=250 g
During 4 th half-life,mass of Sr-90= 250/2=125 g
Hence, after 116 years 125 g of strontium-90 is left out.
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A half life of 29 years means that after 29 years, only half of this isotope would be left due to radioactive decay. If we start of with 2000g of it and leave it for 116 years, it would undergo 4 "half=lives". 116/29 = 4. So after the first half life, we would have 2000g/2 = 1000g left. After the second, we would have 500g left, after the third, 250g so after 4 half lives or 116 years, there would be 125 g of Strontium-90 left at the site.
