Thread: Radioactivity 1

Hi!
I would be grateful if somebody could help me with this:
"A piece of Am-241 has a radioactivity of 10kBq. Determine how much Am-241 it contains."
Attempt at a solution:
Let A be the activity
Let N be the number of atoms
We know that We can set our initial time to zero, which gives us .
Furthermore, we know that A(t)=-N(t). We also know that . Now we use that one atom weights 241,0568229u and that the half-life is 432,2 y (I converted it to seconds). Then we get that the mass of our "piece" is approximately 78,7 micrograms. However, in the key it says 1,83 ng.
What am I doing wrong here?

When you converted to seconds did you inadvertently multiply by 432.2 twice?

No, I'm afraid I didn't.