Part A) You are using a Geiger counter to measure the activity of a radioactive

Question

Part A)
You are using a Geiger counter to measure the activity of a radioactive substance over the course of several minutes. If the reading of 400. counts has diminished to 100. counts after 72.7 minutes , what is the half-life of this substance? Express your answer with the appropriate units.

Part B)
An unknown radioactive substance has a half-life of 3.20 hours . If 11.3 g of the substance is currently present, what mass A0 was present 8.00 hours ago? Express your answer with the appropriate units.

Part C)
Americium-241 is used in some smoke detectors. It is an alpha emitter with a half-life of 432 years. How long will it take in years for 45.0 % of an Am-241 sample to decay? Express your answer with the appropriate units.

Part D)
A fossil was analyzed and determined to have a carbon-14 level that is 40 % that of living organisms. The half-life of C-14 is 5730 years. How old is the fossil? Express your answer with the appropriate units.

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Explanation / Answer

Part A :

ln (Nt/No) = -kt

or, ln (100/400) = -k * 72.7 min

or, k = 0.0191 /min

t1/2 = 0.693/0.0191 /min = 36.34 min

Half life of this substrance is 36.34 min

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