2. a 2.5 gram sample of an isotope of strontium-90 was formed in a 1960 explosion
of an atomic bomb at johnson island in the pacific test site. the half-life of
strontium-90 is 28 years. in what year will only 0.625 grams of this strontium-90
remain?

2016

Explanation:

In order to solve this problem, firstly we have to use the Radioactive Half Life Formula:  

(1)  

Where:  

is the final amount of strontium-90

is the initial amount of strontium-90

is the time elapsed  

is the half life of strontium-90

Knowing this, let's substitute the values and find from (1):

(2)  

(3)  

Applying natural logarithm in both sides:

(4)  

(5)  

Clearing :

(6)  

(7)  This is the time elapsed since the sample of strontium-90 was formed

If we add this time to 1960 (the year this isotope was formed) we finally find the year when only 0.625 grams of strontium-90 are left:

Therefore, the answer is 2016.