# Help calculating the the activity of a radioactive sample

1. Nov 8, 2008

### pearlblack

Help!!! calculating the the activity of a radioactive sample!!

1. find activity R

2. the average 70 KF males contain 140 gram of potassium. of this 0.012% is potassium 40. the half life of potassium 40 is 1.2 billion years. so how many K40 decay per second and what is the activity of K40 in the body??

3. so my solution here is..
140g = 0.14kg
and mass of K40 = 0.14 * 0.012/100 = 1.68 * 10^-5kg
how many decay per second = (1.68*10^-5)/2 / 3600*24*365*1.2*10^12 = 2.22*10^-25kg
then i use R = $$\lambda$$ * N = 0.653N/half life
= 3.88 * 10^-45 Bq
but this answer is wrong!!! cuz the correct one is 4200 Bq
so help me plz!!! thx

2. Nov 8, 2008

### joeyar

Re: Help!!! calculating the the activity of a radioactive sample!!

Okay, you've got the mass of K40 = 1.68*10^-5 kg. You need to calculate how many K40 atoms there are by multiplying by Avogadro's number and dividing by the molar mass (which is 40).

3. Nov 9, 2008

### pearlblack

Re: Help!!! calculating the the activity of a radioactive sample!!

sorry i dont get it..
do i just
1.68*10^-5kg * 6.02*10^23/40 = 2.5284*10^17
and then sub this into the equation R=0.653N/half-life??
but the answer i get from this is 4.36*10^-3

which i think there is still something wrong in ym calculation....