If 8 g of a radioactive substance are present initially and 7 yr later only 4 g remain, how much of the substance will be present after 8 yr?

Answer 1

Since the amount dropped to 1/2 of the initial amount over a period of 7 years, you can assume the half-life is 7 years.

m(t) = m₀ (0.5)^t/7,

t = years elapsed from the time the amount was m₀

In grams,

m(t) = 8 (0.5)^t/7

m(8) = 8 (0.5)^8/7 g ≅ ? g