Question 691570
The mass m(t) remaining after t days from a 25-g sample of thorium-234 is given by 
m(t)=25e^(-0.0277)t 
a) How much of the sample will remain after 70 days? Round the answer to the nearest integer. (Answer should be in terms of grams)
m(70) = 25*e^(-0.0277*70) = 3.5962 grams
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b) After how long will only 10g of the sample remain? Round the answer to the nearest integer. (Answer should be in terms of days)
25*e^(-0.0277t) = 10
e^(-0.0277t) = 2/5
-0.0277t = ln(0.4)
t = 33.079 days
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c) Find the half-life of thorium-234. Round the answer to the nearest integer. (Answer should be in terms of days)
e^(-0.0277t) = 1/2
-0.0277t = ln(.5)
t = 25 days when rounded down
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Cheers,
Stan H.