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

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 51.9 minutes , what is the half-life of this substance?

t1/2=__value_____ ______units_____

Part B

An unknown radioactive substance has a half-life of 3.20 hours . If 12.1 g of the substance is currently present, what mass A0 was present 8.00 hours ago?

A0=_____value___ ____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 32.0 % of an Am-241 sample to decay?

t=______value_____ ____units_______

Part D

A fossil was analyzed and determined to have a carbon-14 level that is 50 % that of living organisms. The half-life of C-14 is 5730 years. How old is the fossil?

t=______value_____ _____units_______

Explanation / Answer

part A

K = (1/T)ln(a/x)

   = (1/51.9)ln(400/100)

   = 0.0267 min-1.

t1/2 = 0.693/k = 0.693/0.0267 = 25.955 min

part B

K = (1/T)ln(a/x)

(0.693/3.2) = (1/8)ln(a/12.1)

a = 68.42 grams

part C

K = (1/T)ln(a/x)

(0.693/432) = (1/t)ln(100/68)

t = 240.413 years

part D

t1/2 = 5730 years

so that

t = 5730 years.

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