Posted by lolo on Monday, August 22, 2011 at 8:49pm.
How long will it take for the 100mg of cesium137 to decay to 6.25mg(1/16 the original amount)?

chemistry  DrBob222, Monday, August 22, 2011 at 8:58pm
2^{n} = 16
Solve for n = number of half lives.

chemistry  lolo, Monday, August 22, 2011 at 9:59pm
still i can't understand it

chemistry  DrBob222, Monday, August 22, 2011 at 10:31pm
Do you have the halflife given in the problem or are you allowed to look it up? If so there is an easier way to set it up.
To solve 2^{n} = 16, take the log of both sides.
log 2^{n} = log 16
n*log 2 = log 16
n*0.301 = 1.2 etc.
There is another way. Remember that halflife means the time it takes for 1/2 the material to disappear. So make a table like this. The periods are used for spacing.
amount......# halflives
100 mg.........0
50 mg..........1
25 mg..........2
12.5 mg........3
6.25 mg........4
So whatever the halflife is, multiply that by 4 and you will have the time it takes to go through 4 halflives.

chemistry  Anonymous, Tuesday, August 23, 2011 at 1:24am
thanks alot , this answer really helped me to understand this question
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