Posted by UCI student on Friday, April 10, 2009 at 3:10am.
i think you have omitted part of the procedure but the following may get you started. For the standard:
Fe^+3 + SCN^- ==> FeSCN^+2
We had 9.00 mL 0.200 M Fe^+3.
1.00 mL 0.002 M SCN^-
Clearly, SCN^- is the limiting reagent and the iron(III) is in excess.
moles FeSCN^- formed = M x L = 0.002 M x 0.001 L = 2 x 10^-6 moles.
What is the concn? M = moles/L. You have the 2 x 10^-6 moles in 10 mL (it was diluted 1:10) so the M = 2 x 10^-6/0.010 = 2 x 10^-4 M.
The next step is to evaluare a in the equation A = abc where A = absorbanace, a is the absorptivity constant, b is the cell length but we will ignore that since you are using the same cell (or at least the same length cell) in both standard and sample alike, and c is the concentration. [Technically, the constant a is epsilon and is the molar absorptivity constant when c is measured in moles/L but we'll just continue to call it a).
So A = a*c
0.510 = a*2 x 10^-4
a = 2.55 x 10^3 but check confirm this yourself.
Now we get to the point that I don't know how the trial run was treated. hope this is enough to get you started.
yea, that's about as far as i got. then i tried an ice table, but im not getting the right answers still
You missed my first pick-up line. I don't know how the "trial runs" were treated. I assume you mean by "trial's absorbance" the absorbance of the unknown sample. How was the sample treated before the absorbance was measured?
haha, thanks.
turns out I was just formatting my answer wrong each time, I get it. nvm