Analytical Chem Lab 2 - Simultaneous determination of permanganate and dichromate

For the 4 graphs, we should plot absorbance against volume (ml)
We can do this because the volume is proportional to concentration.
This produces a way different epsilon for the solution and that's okay.
Epsilon units change. Epsilon = A / (ml x cm)

We use the slope of m in the line equation of each graph as the epsilon to plug into the 2 equations for solving 2 unknowns. y = mx + b

We can find an example of how to do this in Harris on p. 422, (It's long and uses substitution method)
OR
We can work this like the problem that was done in class, problem 19-9 in Harris, if we got it all in class notes.

The answers from solving 2 equations for 2 unknowns will be the answer in ml of stock solution, not concentration.
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Resist the temptation to find ml of stock by plugging Absorbance of unknown into the line equations for the graphs. Each graph is only one part of a bigger picture that requires all 4 graphs, hence the 2 equations for solving 2 unknowns.

So the answer we get after solving the 2 equations is in mL not Molarity? I ended up with 3.07 for one and 8.09 for the other. I wasn't sure if I needed to do anything else with these or if I was done.

That's it. You are done.
Mine were 2.84 ml for one and 8.00 ml for the other.
The solution was red, with a slight purple tint. Unknown #10.

So, what is our conclusion from this lab from this information????

Did you plot the blank in each of your graphs ?
conclusion is that we successfully determined the amount of 2 components in a mixture, simultaneously, using UV-vis spectroscopy and applying Beer-Lambert's law.

I just have one line in each of the four graphs. Nothing with the blank.

Also, how do we know which component goes with which concentration?

If the equations are set up like the example on page 422, Harris
your solutions will be whichever one you plugged into X or Y

I sent an email to him about what to call this modified molar absorptivity.
I know it's experimentally the same concept, but since the units changed I don't know that we should still call it molar. I'm more comfortable with something like Standard Absorptivity of 0.015 M ...

I will post when I hear back.

RE: including the blank
What I meant was did you list data volume like this?:
0.00
2.00
4.00
etc

My questions and the responses:

1) It seems to make only a slight difference when graphing, if I include the blank (0,0) on the graph.
Is there a preference for whether 0 volume (or concentration) data is graphed?

RESPONSE
That's a good sign. Your data should want to trend through 0,0.
That's the expected Beer's Law behavior. There is no preference, but
like I said before, if you plot the mL of stock as the x-axis, you
will not need to do unit conversions when you solve for x and y.
Not exactly what I meant to ask but I think I will leave that data point off.

2) Plotting Absorbance vs Volume makes titling the graph a bit tricky.
Is epsilon still properly called "molar" absorptivity? I know the graphs reflect molar absorptivity but the units change to 1 / (ml*cm).
"Standard Absorptivity for 0.0150 M Potassium Dichromate at 440 nm" seems to be a better title than "Molar Absorptivity of Potassium Dichromate at 440 nm." I understand the concepts but I'm looking for your preference.

RESPONSE
2) You are correct; if the mL of stock are plotted instead of the
molarity, then the units for epsilon do change. You are technically
finding the "absorptivity" rather than the "molar absorptivity." You
can and should still use the absorptivity for your algebraic
equations. The title of your graph should not be "molar absorptivity"
because you are not plotting the molar absorptivity. You are doing a
Beer's Law Graph, so you can call it exactly that, along with the
additional identifying information (for Mn or Cr at ___ nm.)

Thanks for posting those responses. I did not post through 0. I started at 2 ml on all graphs.

Are we supposed to evaluate our graphs in some way?

Actually, we can do this by using the concentration values on X-axis.
He told me yesterday we couldn't do it because we didn't know the volume of the mixture.
We CAN relate moles per liter to moles per liter.
I did these plots both ways and got the same answers. I don't know why because I feel like my unknown was more than 100 ml.
Solve for the volume by c1v1 =c2v2
I wouldn't trust C1V1 without knowing volume of Unknown