Posted by lolo on .
How long will it take for the 100mg of cesium137 to decay to 6.25mg(1/16 the original amount)?

chemistry 
DrBob222,
2^{n} = 16
Solve for n = number of half lives. 
chemistry 
lolo,
still i can't understand it

chemistry 
DrBob222,
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,
thanks alot , this answer really helped me to understand this question