Posted by Sally on Monday, January 28, 2013 at 10:26am.
how many half lives have passed if only 0.625g remain of the original 40g?

Chemistry  Steve, Monday, January 28, 2013 at 10:54am
(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, Monday, January 28, 2013 at 3:28pm
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
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