Question

5. The isotope neptunium-238 has a half-life of 2.00 days. If 4.0 grams is the starting amount, how much will be left after 12 days?

Answer 1

The radioactive decay law says

$$N(t)=N_02^{-t/t_{1/2}}$$

Hence, the amount of isotope neptunium-238 that left after 12 days

$$m=(4.0\:{\rm g}) \times 2^{-12/2}=0.0625\:\rm g.$$