In my experiment, I was given cobalt (II) acetate with an unknown molarity and I'm supposed to determine its molarity. I used a spectrometer to find out the absorance of the solution with the unknown molarity, and I just assumed I could find the coefficient online somewhere and thus use Beer's Law to calculate it pretty easily. I could not find it anywhere, and I realized I should have experimentally determined the extinction coefficient (that's what I meant by "derive" in my last post) and it's too late to do that now.
I appreciate the reply, hope that clarifies some things