The halflife for the radioactive decay of U238 is 45 billion

The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration.

B)If a sample of U-238 initially contained 1.9×10^{^18} atoms when the universe was formed 13.8 billion years ago, how many U-238 atoms will it contain today?

Solution

ln(Co/C)=k*t

k=ln(2)/half-life

A)ln(Co/0.85Co)=k*t

t=1.055billion years

B)

ln(1.9*10^(18)/N)=K*13.8

N=2.2677*10^(17) atoms