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**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 = [tex]\lambda[/tex] * 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

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 = [tex]\lambda[/tex] * 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