Krypton-85 has a half-life of 10 years. What percentage of ^{85}Kr produced from nuclear testing in 1959 still remains today?

25.0%

3.13%

12.5%

6.25%

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Method 1:

The half life formula: A = Ao(2^(-t/h) whereAo = original amtt = timeh = half life of the materialA = resulting amt

so, A=Ao(2^(-50/10))/Ao*100=3.125 % ~ 3.13 % Answer

Given Half life, t_{1/2}=10yrs

t_{1/2}=0.693/k

k=0.693/10=0.0693 yrs^{-1}

Now let [Kr]_{0} be initial concentration in 1959, and [Kr] be conc today.

Time, t=2014-1959=55yrs

ln([Kr]_{0}/[Kr])=kt

[Kr]_{0}/[Kr]=e^{0.0693*55}

[Kr]_{0}/[Kr]=45.218

[Kr]=0.022[Kr]_{0}

thus % remaining in 2014 is 2.2%

If the ques would have been solved in 2009:

Percent remaining=3.13% Answer

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