Question

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

b)An unknown radioactive substance has a half-life of 3.20 hours . If 14.6 g of the substance is currently present, what mass A0 was present 8.00 hours ago?

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 30.0 % of an Am-241 sample to decay?

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?

Answer 1

o A= Initial final count 7 count ] 7 ln (400) KX 76-2 loo KE 0.0182 mint tya = 0.693 = 0.693 = 38.00 K 0.0162 b) ( 0 ,. 0.699 , 01215 N ti2 3.20 In(A) = kt => In ( 10 ) - 0.216688.00 1 14.6) Ao = 82 5845 gramy 19 Ka 0.693 0.693 = 1.6oux 10 yeart o tya 432 Ao= 100% , A= 30% en/100) = 1.604 x 103 xt += 750.61 year d) Ao=100% , A = 50% K= 0.69 0.693 = 1.21x10 yeart 5730 1/2 In ( A ) = x + a ln(100) = 1.218 104 x t += 5728.49 years

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