Question 1113943
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The half-life of a particular radioactive isotope is 72 hours. 
Write an equation to relate the mass of radioactive material remaining to time (in hours). 
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{{{y=pe^(-kt)}}}

{{{ln(y)=ln(p)+ln(e^(-kt))}}}

{{{ln(y)=ln(p)-kt}}}

{{{ln(y)-ln(p)=-kt}}}

{{{kt=ln(p)-ln(y)}}}


{{{k=(1/t)(ln(p/y))}}}


If half life is 72 hours, then {{{k=(1/72)*ln(2)}}}

{{{k=0.009627}}}


This model then  becomes  {{{highlight(y=p*e^(-0.009627t))}}}
p, initial quantity
y, final quantity
t, time in hours


You can figure the other questions based on that.