An isotope of lead, 201Pb, has a half-life of 8.4 hours. How many hours ago was there 20% more of the substance?

Answer 1

M = M₀e^rt;, if t = 8.6 then M = M₀/2; M₀/2 = M₀e^8.4r; e^8.4r = 1/2 r = - ln2/8.4 = - 0.08252

M = M₀e^-0.08252t; 1.2M₀ = M₀e^-0.08252t; t = ln1.2/(- 0.08252) = - 2.209 hours (ago)