Check out some similar questions!

heyyalliee · May 4, 2011, 11:57 AM

Half life of 15 over 8 Oxygen is 122 seconds.

How long does it take for the number of 15/8 O nuclei in a given sample to decrease by a factor of 1Ã—10âˆ’4?
Answer in min.

I don't even know where to start. I have formulas but I don't know how to use them properly.
Any help is awesome!

Unknown008 · May 4, 2011, 12:08 PM

I use the formula:

$A_t = A_o $\frac12$^{t/h}$

I can't see your factor clearly... :(

Anyway, from what is given, the half life is 122 s

The factor is given by At/Ao

So that you get:

$Factor = $\frac12$^{t/h}$

where h is the half life, t is the time in second you're looking for which you then need to convert into minutes, Factor is the factor given.

heyyalliee · May 4, 2011, 12:25 PM

sorry factor did get all messed up... it was 1*10^-4
So I can do:
1*10^-4=(1/2)^t/h ?

heyyalliee · May 4, 2011, 12:30 PM

I haven't done this in so long even my basic math sucks.. How can I get t by itself?

Unknown008 · May 5, 2011, 08:50 AM

Okay, insert log

$\log (factor) = \log $\frac12$^{t/h}$

$\log (factor) = \frac{t}{h} \log $\frac12$$

$\frac{\log (factor)}{\log $\frac12$} = \frac{t}{h}$

$t = h \frac{\log (factor)}{\log $\frac12$}$