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Topic: I don't understand Beer's Law plot. How do I get the concentration?  (Read 18100 times)

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Offline toadesque

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I did a lab and I have to get the concentration of an unknown Copper (II) solution.

I have the absorbance which would be my y axis but I don't have the concentration which would be my x axis. Where would I get the concentration from? Also what is my calibration curve?

All I have for data is the volume of unknown solution used. My lab is also asking me for the moles of Cu (2+). How would I get that???? I don't have any masses (grams)!!! I don't even have the molarity so how do I get the moles??

Offline Borek

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #1 on: December 09, 2008, 03:02:23 AM »
Sounds like you are missing something. Please elaborate on the procedure and information given.
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Offline ARGOS++

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #2 on: December 09, 2008, 04:08:26 AM »
Dear toadesque;

It’s correct, that you need first a calibration. How can you to do that?

Good Luck!
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Offline JGK

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #3 on: December 09, 2008, 11:27:07 AM »
Were you given any molar extinction coefficient data?
Experience is something you don't get until just after you need it.

Offline toadesque

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #4 on: December 11, 2008, 03:32:19 AM »
Okay I did a lab and it's asking me to find the molarity of an unknown Cu (II) solution.

I first did it for a known copper solution. I am supposed to use the data I got from here to help get the molarity of the unknown.

I have to make a Beer's Law plot to do this. On the y axis I have the absorbance which I obtained from using a spectrophotometer. On the x-axis I have the concentration. But the problem is that I don't know how to get the concentration.

Here is the data I have on the known copper solution

Volume of stock solution used: 1.03 mL.
Moles of Cu (2+) used: 1.2875 x 10^-4. I obtained this by using the given molarity of the stock solution (.125 M) and using the volume of stock solution used.

OK. So If I can get the concentration for the graph then I can finish my lab easily.

I first tried to get the concentration by doing:

1.2875 x 10^-4 moles of Cu / 1.03 mL

I got 1.25 x 10^-4

So i thought I had it down correctly. Then I proceeded to do the same for my 2 mL solution, 3 mL solution, 4 mL solution, etc and the molarity came out the same every time. So that doesn't make sense at all. I don't know what else to do.

Offline toadesque

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #5 on: December 11, 2008, 03:34:09 AM »
just a note, i left out the data for my 2-5 mL solutions. Once I know how to do it for 1 I can do the rest.

Offline ARGOS++

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #6 on: December 11, 2008, 03:59:06 AM »

Dear toadesque;

If I understand you correctly:
Now we know the individual amount of stocksolution you were taken, but we don’t know the individual end volume of each sample.
Without you are not able to calculate the final concentration of each sample.

If I assume that the end volume was always the same the final concentration ratio must follow the used volume of the stocksolution.

Good Luck!
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Offline toadesque

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #7 on: December 11, 2008, 04:09:40 AM »

Dear toadesque;

If I understand you correctly:
Now we know the individual amount of stocksolution you were taken, but we don’t know the individual end volume of each sample.
Without you are not able to calculate the final concentration of each sample.

If I assume that the end volume was always the same the final concentration ratio must follow the used volume of the stocksolution.

Good Luck!
                    ARGOS++



Not sure what you mean by end volume of each sample.

All I did was titrate 1 mL of stock solution, then 2, then 3, then 4, then 5. Added 20 mL of NH3 to all of those and then filled the remainder up with DI water to 100 mL mark. But don't think that's necessary to know.

When I make my graph, I will have the absorbance on the y axis. The concentration of the known on the X. When I draw a straight line, I can then look at the absorbance and line it up with the concentration axis to get the concentration, then I just do M1V1 = M2V2 to get the concentration after diluted.

*EDIT

Erm....I think maybe I know what you're talking about. That 100 mL actually was important wasn't it?

.125 X volume of stock solution divided by 100 mL = concentration

Is that right?
« Last Edit: December 11, 2008, 04:25:14 AM by toadesque »

Offline ARGOS++

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #8 on: December 11, 2008, 04:47:19 AM »

Dear toadesque;

So your calibration concentrations are  0.00132 ;  0.002575;  0.0038625; and  0.00515 molar.

Then you can do the plot.
After the plot created you can use the absorption of the unknown sample and read its concentration from the plot.

So I don’t see any problem at the moment. Can you tell me more about?

Good Luck!
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Offline toadesque

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #9 on: December 11, 2008, 04:56:44 AM »
How did you get those numbers? I was getting something completely different.

My exact stock solution volumes are

1.03
2.01
2.98
4.0
5.03

But I'm confused as to how you got those?

Offline ARGOS++

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #10 on: December 11, 2008, 05:12:26 AM »

Dear toadesque;

So you can do the calculation by yourself:

Conccalib = concstock * volstock / 100.0 ml

So for 1.03 ml:  conccalib = 0.125 M x 1.03 ml / 100.0 ml =  0.0012875 M
(It's normal dilution.)


Good Luck!
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Offline toadesque

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #11 on: December 11, 2008, 05:18:55 AM »
When I'm plotting this, do I just use the absorbance of the known solution? Otherwise I don't have an X axis to go along with those concentrations to plot them. And then use the absorbance for the unknown solution to get the unknown conc.

Also I'm not getting a straight line if I do it that way.....possibly wrong then?

Offline ARGOS++

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #12 on: December 11, 2008, 05:28:55 AM »

Dear toadesque;

Yes!, - you have to plot known conc. vs. known absorptions.
Then construct the straight line.
And now you can read the unknown concentration.

Otherwise you have to give all known data.

Good Luck!
                    ARGOS++

Offline toadesque

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #13 on: December 11, 2008, 05:32:31 AM »
:( I'm not getting a straight line, it looks so ugly it's bending everywhere.

It must mean that my absorbances were wrong when I got them and it's ruined my entire lab! And it will ruin my unknown absorbances as well.

When I did the spectrophotometer, something weird happened and the absorbance for my 4 mL solution ended up being LOWER than the absorbance for the previous 3 so that ruined all my data. Is there anything I can do to fix this? Like just change my absorbance for the 4 mL so that it looks semi appropriate?

Offline ARGOS++

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Re: I don't understand Beer's Law plot. How do I get the concentration?
« Reply #14 on: December 11, 2008, 05:38:19 AM »

Dear toadesque;

As a first approach you should skip the values for 4 ml, and try again.

Good Luck!
                    ARGOS++


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