Cesium-137 has a half-life of 30 years. If a 2.0 g sample of cesium-137 decays over 120 years, how much would remain?

Question

Quizplus · Accepted Answer

Since cesium-137 has a half-life of 30 years, this means that half of the sample will disappear every 30 years.
Over a period of 120 years, cesium-137 will undergo 4 half-lives (120/30 = 4).
So, to get the final answer, we divide 2.0g in half four times:
30 years: 2 ÷ 2 = 1
60 years: 1 ÷ 2 = 0.5
90 years: 0.5 ÷ 2 = 0.25
120 years: 0.25 ÷ 2 = 0.125
So after 120 years, only 0.125 g would remain.

Cesium-137 Has a Half-Life of 30 Years