Understanding Beer's Law and Calculating Absorbance

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Discussion Overview

The discussion revolves around understanding Beer's Law and calculating absorbance in the context of a homework problem. Participants explore the relationship between concentration and absorbance, as well as the implications of transmittance on these calculations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the units and the meaning of εb in Beer's Law, questioning if it relates to % transmittance.
  • Another participant speculates that the question involves determining the concentration of an unknown solution based on its transmittance.
  • A different participant asserts that εb is not necessary for constructing a calibration curve and emphasizes the linear relationship between absorbance and concentration.
  • One suggestion involves plotting concentration against transmittance on a semi-log scale to derive a linear relationship for calibration.
  • A participant reports finding absorbance values using the formula A=log(1/%T) but notes that their results yield a negative slope, which contradicts expectations.
  • Another participant requests to see the graph of log T as a function of concentration to further analyze the results.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the application of Beer's Law, particularly in relation to the expected outcomes of their calculations. There is no consensus on the interpretation of the results or the correct approach to the problem.

Contextual Notes

Some participants mention specific absorbance values and the unexpected negative slope of their graphs, indicating potential issues with their calculations or understanding of the relationship between transmittance and absorbance.

Feodalherren
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Homework Statement



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The Attempt at a Solution



So I'm at a loss here. I'm not given any units... I know the concentration is C in Beer's law (awesome name btw). But what is my εb? Is it % transmittance?

So then for A

A= (.055)(.0001)
What does this even mean?Ops this was supposed to go to the chemistry forums.
 
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What is the question they are asking? Incidentally, Beer is the guy's name.

Let me guess the question. They give you the transmittance of an unknown solution, and they want you to find its concentration.

Chet
 
εb doesn't matter here.

This is about constructing calibration curve, C vs A. Form the Beer's law (AKA Lambert-Beer's law) you know the dependence is linear (A=kC), so it is enough to determine the value of k - and it doesn't matter what are its components.
 
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Have your graphics program plot the concentration (y axis) as a function of the transmissivity (x axis). Have your graphics program use a semi-log scale for transmissivity). The graph will come out to be a straight line. Have your graphics program fit an equation to this straight line. This will express the concentration as a linear function of log of transmissivity. This will constitute your calibration.

Chet
 
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That's where I kind of get lost.
So my Cs are obviously given. Now I found a formula for A which was
A=log(1/%T) so I found my absorbance numbers.

Thing is, they increase with a lower T as shown by the above equation. I should get a line that has a y-intercept of approximately 0, my line goes the wrong way, it has negative slope :/.
 
Feodalherren said:
That's where I kind of get lost.
So my Cs are obviously given. Now I found a formula for A which was
A=log(1/%T) so I found my absorbance numbers.

Thing is, they increase with a lower T as shown by the above equation. I should get a line that has a y-intercept of approximately 0, my line goes the wrong way, it has negative slope :/.
What does your plot of log T as a function of C look like? It should be a straight line.

Chet
 
It is a straight line but the slope is negative and it has a big y-intercept...

I get absorbances to be the following
A 1.3
B .646
C .335
D .165
E .082
 
Feodalherren said:
It is a straight line but the slope is negative and it has a big y-intercept...

I get absorbances to be the following
A 1.3
B .646
C .335
D .165
E .082
Is it possible to show us your graph?

Chet
 

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