Part B An unknown radioactive substance has a half-life of 3.20hours . If 46.2g

Question

Part B

An unknown radioactive substance has a half-life of 3.20hours . If 46.2g 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 34.0% 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 80% 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.

Explanation / Answer

B. Half Life = 3.2 hours

Time Elapsed = t = 8 hours

Mass left = N = 46.2 g

t1/2 = 0.693/k

k = 0.693/3.2 = 0.2166 h-1

N = N0e-kt

N0 = 261.25 g

C. t1/2 = 432 years

k = 0.693/432 = 0.0016 year-1

N/N0 = 1 - (34/100) = 0.66 = e-kt

kt = -ln(0.66)

t = 259.022 years

D. Half Life = 5730 years

k = 0.693 / 5730 = 1.209x10-4 year-1

N/N0 = 0.8 = e-kt

t = 1845.04 years

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