The half life for radioactive decay (1st order) of plutonium 239 is 24,000 years. How many years would it take for 1 mole of this radioactive material to decay so that just one atom remains?

Well, how many atoms in a mole? Avogadro's number. Start with that. PS You will be very old!

See, the governing equation for 1st order kinetics is :-
k*t = ln{[A0]/[A]}; where k = rate constant = ln2/half life ; t = time interval in which the concentration of the reactant changes from [A0] to [A].

Use the above eqaution to get your result.

Revert with queries, if any.