Calculating activity using half life

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To calculate the activity of potassium-40 (40K) in a 75 kg adult, the initial calculations included determining the decay constant and the number of atoms of 40K in the body. The user calculated a decay constant of 6.18x10^-14 but mistakenly used 14.11 kg for potassium instead of the correct amount based on the 0.2% potassium content. The discussion raised questions about the accuracy of the mass of 40K in the body and the desired units for the final activity result. Clarifications on these points are essential for arriving at the correct activity value, which is expected to be around 4.39x10^-6. Accurate calculations depend on understanding the correct proportions and units involved in the activity formula.
Kira127
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The question is that the human body contains 0.2% potassium by weight and the natural abundance of 40K is 0.0118%. The half life of 40K is 1.28x10^9 years and I have to calculate the activity in an adult weighing 75 kg. What I tried was to find the decay constant by 0.693/1.12x10^13 hours = 6.18x10^-14. Then I took the mass and divided it by the 40K 14.11kg/40 * 6.02x10^23 = 2.12 x10^23. Then I tried Activity=lambda*N =6.18x10^-14*2.12x10^23=1.31x10^10. The answer is supposed to be 4.39x10^-6 and I don't know what I'm doing wrong if someone could help me that would be very much appreciated.
 
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A couple of questions:

(1) If you weigh 75kg, and 0.2% of that is K, and 0.0113% of THAT is K40, do you really think there are 14kg of K40 in your body?

(2) Do you really want the time constant in 1/hr?

(3) If you think the final answer is 4.39E-6, what are the units of that?
 

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