The half life of hydrogen-3 is approximately 12 years. In how many years will half life of a 20 gram sample decay by beta radiation to helium-3?

First, you need to solve for the reaction constant k. Radioactive decay is a first order reaction. You will use this equation to solve for k.

k = ln (2) / half-life = 0.639/12years = 0.05325

Next, plug your values into the first order integrated rate law. The rate law has two common forms:

ln (A]t / [A]0) = - kt and [A]t = [A]0 * e ^ (-kt)

Your problem requires the use of the first equation.

Rearrange the first equation so you get

(-1/k) ln (A]t / [A]0) = t

t = 35.6 years