posted by Anonymous .
How long will it take for 25% of the C-14 atoms in a sample of C-14 to decay? The half-life for the radioactive decay of C-14 is 5730 years.
If a sample of C-14 initially contains 1.1 mmol of C-14 , how many millimoles will be left after 2275 years?
Both problems are similar. Use this first equation to evaluate k.
k = 0.693/t1/2
Then substitute k into this equation.
ln(No/N) = kt
To make thing easy I would assume we start with 100 atoms so No = 100. If 25% of the atoms decay, that will leave 75 atoms remaining; therefore, N = 75. You know k from the first equation, you can solve for time.
Second problem is done the same way but you solve for N.