An isotope of Cesium -137 has a half-life of 30 years. If 1.0 g of Cesium – 137 disintegrates over a period of 60 years, how many grams of Cesium- 137 would remain?

Explanation:

Radioactive decay follows the equation:

Ln [A] = -kt + ln [A]₀

Where [A] is amount of isotope after time t: Our incognite,

k is rate constant: ln 2 / Half-life = 0.0231 years⁻¹

t are 60 years

[A]₀ is initial amount of isotope: 1.0g

Replacing:

Ln [A] = -kt + ln [A]₀

Ln [A] = -0.0231 years⁻¹*60 years + ln 1.0g

ln [A] = -1.386

[A] = 0.25g