Question

100g of radioactive element X are placed in a sealed box. The half-life of this isotope of X is 2 days. After 4 days have passed, what is the mass of element X in the box?

b) If the box is completely evacuated (perfect vacuum), then N atoms of X are added. After 4 days have passed, how many atoms are in the box?

Please help!! Thanks

Answer 1

4 days is 2 half-lives, so 1/4 is left

Answer 2

For b) would the answer be the same number of N atoms of X, because the box is in a perfect vacuum?

Answer 3

the vacuum does not matter. After 4 days, there will be 1/4 of the starting amount left.
I guess the vacuum is provided to be sure there are no residual atoms of X hiding there.