Q21 of 94 Page 442

Radioactive 131I has a half-life of 8.0 days. A sample containing 131I has activity 20 μ Ci at t = 0.

(a) What is its activity at t = 4.0 days?


(b) What is its decay constant at t = 4.0 days?


Given,

Half-life = 8 days, A0 = 20μCi


a) We know that,




Where,


A= Activity of the substance


A0 = Initial activity


t = time


λ = decay constant


So,







b) Decay constant in per second s-1



More from this chapter

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19

: The decay constant of (electron capture to ) is 1.8 × 10–4 s–1.

(a) What is the half-life?


(b) What is the average-life?


(c) How much time will it take to convert 25% of this isotope of mercury into gold?


 20

The half-life of 198Au is 2.7 days.

(a) Find the activity of a sample containing 1.00 μg of 198Au.


(b) What will be the activity after 7 days? Take the atomic weight of 198Au to 198 g mol-1.


 22

The decay constant of 238U is 4.9 × 10–18 s–1.

(a) What is the average-life of 238U?


(b) What is the half-life of 238U?


(c) By what factor does the activity of a 238U sample decrease in 9 × 109 years?


 23

A certain sample of a radioactive material decays at the rate of 500 per second at a certain time. The count rate falls to 200 per second after 50 minutes.

(a) What is the decay constant of the sample?


(b) What is its half-life?