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Beers Law?

  1. Sep 27, 2009 #1
    1. The problem statement, all variables and given/known data
    I'm given the mass of an unknown sample, and the absorbance of said unknown sample. Using that and the beers law graph slope of 0.22L/mg of IronII and a y intercept I need to find the percent of the Iron II ion in the sample.


    2. Relevant equations
    I've never heard of beers law before...I think this is the equation, but I have no clue how to use it, or which one from here to use.

    http://en.wikipedia.org/wiki/Beer–Lambert_law


    How does this law work, and how can I go about doing this calculation?
     
  2. jcsd
  3. Sep 27, 2009 #2

    Borek

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    Staff: Mentor

    Basically concentration is directly proportional to absorbance, that's all you need.

    c = intercept + k*absorbance

    In theory intercept should be 0, in practice it is not always the case.
     
    Last edited by a moderator: Aug 13, 2013
  4. Sep 27, 2009 #3
    So C is the concentration, the intercept is the absorbance when the concentration is zero, and absorbance is...the absorbance of the unknown. Right.

    Then is K the slope?
     
  5. Sep 27, 2009 #4

    Borek

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    Staff: Mentor

    Yes. It should be absorbance coefficient times path length, but when doing experiment we usually prepare calibration curve and we don't pay attention to details, we are just interested in slope, as that's enough.
     
    Last edited by a moderator: Aug 13, 2013
  6. Sep 27, 2009 #5
    Path length? Is that the distance it travels through the sample? Also how does the mass of the sample factor into this? I'm sure we were given it for a reason.
     
  7. Sep 27, 2009 #6

    Borek

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    Staff: Mentor

    Yes for path length.

    What is definition of percentage?
     
    Last edited by a moderator: Aug 13, 2013
  8. Sep 27, 2009 #7
    It's the fraction of something out of 100 parts...

    Sorry, I'm not really understanding this. :/ If I have the mass and the absorbance average, then I can plug it in to get this:

    concentration of ion in sample = (slope as L/mg of ion)*absorbance(average) + intercept

    So how does the mass of the whole sample factor in to this?
     
  9. Sep 27, 2009 #8

    Borek

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    Now, what is the definition of concentration?

    I am guessing how the experiment looked alike, so I can be wrong. Please outline what you did.
     
    Last edited by a moderator: Aug 13, 2013
  10. Sep 27, 2009 #9
    Well, it's not an experiment. It's a just a question we need to do given that data.

    Concentration is mols per unit of volume...but how does that help? I don't know what percentage of the mass of the sample is the iron, or water.

    Could I divide the slope by the molar mass of iron to get the mols? It is in L/mg.
     
    Last edited: Sep 27, 2009
  11. Sep 27, 2009 #10

    Borek

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    Staff: Mentor

    Do you know volume of the solution? Can you calculate amount of iron?
     
    Last edited by a moderator: Aug 13, 2013
  12. Sep 27, 2009 #11
    No, I don't know the volume.
     
  13. Sep 27, 2009 #12

    Borek

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    Staff: Mentor

    So you can't solve - could be that's the information you forgot to ask for (in case anyone reads it later - we started discussion on chat).
     
    Last edited by a moderator: Aug 13, 2013
  14. Sep 27, 2009 #13
    I actually just had a little breakthrough.

    The information I have:

    slope = 0.2200 L/mg Fe2+
    intercept = .004

    and the info I that lost was the sample mass and the absorbance of the sample.

    So in y=mx+b form the setup goes to:

    absorbance=(0.2200)(concentration of ion in mg/L)+(.004)

    so the absorbance I lost on the way home would go into that, then I can isolate the concentration...and then what?

    I need to find the mass of the Iron. I know the total mass (or rather did till the sheet got trashed), but I don't know the volume...OR DO I!? It says it follows the same procedure as outlined in the lab manual. Give me a little bit, I think all this random bouncing of ideas just paid off. I can just get the volume of the sample from there. After that I can multiply the concentration by that and get the mass.
     
    Last edited: Sep 27, 2009
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