1. An isotope of cesium-137 has a half-life of 30 years. If 5.0 g of cesium-

137 decays over 60 years, how many grams will remain?

1.25 g

Explanation:

Now we have to use the formula;

N/No = (1/2) ^t/t1/2

N = mass of cesium-137 left after a time t (the unknown)

No = mass of cesium-137 present at the beginning = 5.0 g

t = time taken for 5.0 g of cesium-137 to decay = 60 years

t1/2 = half life of cesium-137 = 30 years

Substituting values;

N/5 = (1/2) ^60/30

N/5 = (1/2) ^2

N/5 = 1/4

4N = 5

N = 5/4

N = 1.25 g

Therefore, 1.25 g of cesium-137 will remain after 60 years.