Posted by Anonymous on .
How long will it take for 25% of the C14 atoms in a sample of C14 to decay? The halflife for the radioactive decay of C14 is 5730 years.
If a sample of C14 initially contains 1.1 mmol of C14 , how many millimoles will be left after 2275 years?

chemistry 
DrBob222,
Both problems are similar. Use this first equation to evaluate k.
k = 0.693/t_{1/2}
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. 
chemistry 
Anonymous,
2000 Years