half life

[jd399]

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the half life of a compound is 10 hours, how many hours would it take 72 g to decay to 1/8 of its original activity.. please explain how u get the answer which is 30... explain how to do future problems also please

 

[fsddsms]

the half life of a compound is 10 hours, how many hours would it take 72 g to decay to 1/8 of its original activity.. please explain how u get the answer which is 30... explain how to do future problems also please

(1/2)^n = 1/8 where n = number of half lives
n = 3

1 half life = 10 hours
3 half lives = 3 x 10 = 30 hours

 

[jh311]

This is the way I did it. So.. 1/8 of 72= 9. Start out by making a chart:
You know that every time the sample gets cut in half, 10hrs goes by

72g
↓ 10hrs
36g
↓ 10hrs
18g
↓ 10hrs
9g (this is what you want)

Now just add up the time, or you can just say that each arrow represents the half life time; so the total is 30hrs.

 

[kponenation]

1/2 * 1/2 * 1/2 = 1/8 = (1/2)^3

so 10 * 3 = 30. In this problem the number of grams you have is irrelevant.