Question

The half-life of Sr 90 is 28 years A. how many grams of Sr 90 in...

The half-life of Sr 90 is 28 years
A. how many grams of Sr 90 in a 8.00 g sample of this nuclide will have decayed after 84 years?

B. how many hours are required to reduce the amount of Sr 90 present in 80.0g sample to 10.0mg?

Homework Answers

Answer #1

all radio active decays are first order reactions.

for first order

K = 0.693 / t1/2  = 0.693 / 28

K = 0.02475 y-1

for first order reactions

K = (2.303 / t) log [A0] / [A]

A) 0.02475 = (2.303 / 84) log [8.0] / [A]

0.9027 = 0.903 - log [A]

log [A] = 0.0003

[A] = 1.00 g

after 84 years 8.00 - 1.00 = 7.00 g Sr decayed

7.00 g Sr decayed.

B) [A0] = 80.0 g

[A] = 10.0 mg = 0.01 g

0.02475 = (2.303 / t) log [80] / [0.01]

0.02475 = (2.303 / t) 3.90

2.303 / t = 0.006346

t = 362.9 years = 3179004 hours

answer = 3179004 hours

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