1. May 7, 2015

mss90

1. The problem statement, all variables and given/known data
9.2g of carbon got an activity of 1.6Bq, how old is it?

2. Relevant equations
A = -(lambda)*N
N=No * e ^ -(lambda)*t
(lambda) = ln2/t1/2

3. The attempt at a solution
The question dosnt specify which isotope of carbon it is so halflife remains unknown.
I cant see how to solve this by only knowing the mass and activity.
Could anyone give me pointers?

R

2. May 7, 2015

andrevdh

They are most likely referring to carbon-14 dating.

3. May 7, 2015

mss90

Alright in that case we know the half life and can find the decay constant.
Using A = -(lambda)*N we can find N as well (not sure If I need to divide the activity on 9.2g to get it per grams?)
However It still leaves No and t unsolved.

An alternative method is to; N=No * e ^ -(lambda)*t |*(lambda) => A=Ao * e ^ -(lambda)*t
But it seems to get me to the same "deadend" where Ao and t is unsolved.
I thought I might be able to set t=0 to get Ao but that only makes the expression " e ^ -(lambda)*t" equal 1.
Any tips?

R

PS: thanks for fast reply

4. May 7, 2015

andrevdh

Atoms per mole.
The sample starts out with a certain fraction of it being carbon-14.
Over time the amount of carbon-14 decays.