Posted by Sally on .
how many half lives have passed if only 0.625g remain of the original 40g?

Chemistry 
Steve,
(1/2)^n = .625
n ln(1/2) = ln(.625)
n = .678
makes sense, since one full halflife leave 1/2, which is a bit less than .625 
Chemistry 
DrBob222,
This answer of 0.678 makes no sense to me. By iteration you know that if the sample starts at 40 and ends up at 0.625 it must have gone through six halflives. Mathematically it is done this way.
2^n = 40/0.625 = 64
n*log 2 = log 64
0.301n = 1.8062
n = 1.8062/0.301 = 6.0