An unknown radioactive substance has a half-life of 3.20 hours . If 14.9 g of th

Question

An unknown radioactive substance has a half-life of 3.20 hours . If 14.9 g of the substance is currently present, what mass A0was present 8.00 hours ago?

Express your answer with the appropriate units.

g

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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 35.0 % of an Am-241 sample to decay?

Express your answer with the appropriate units.

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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.

g

Explanation / Answer

An unknown radioactive substance has a half-life of 3.20 hours . If 14.9 g of the substance is currently present, what mass A0was present 8.00 hours ago?

Apply:

A = A0*(1/2)^(t/HL)

14.9 = A0*(1/2)^(3.2/8)

solve for A0

14.9 = A0 * 0.757858

A0 = (14.9)/0.757858

A0 = 19.66 g initially

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 35.0 % of an Am-241 sample to decay?

HL = 432 y

time fo 35 yo decay

Afinal = (1-0.35) = 0.65

so

0.65*A0 = A0*(1/2)*(t/432)

solve for t

ln(0.65)/ln(0.5) = t/432

t = 432*ln(0.65)/ln(0.5) = 268.482 years required

D)

40% of C remaining

0.4*A0 = A0*(1/2)^(t/HL)

ln(0.4)/ln(0.5) = t/5730

t = 5730 * ln(0.4)/ln(0.5) = 7574.647 years

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