Understanding Beer's Law and Calculating Absorbance: A Step-by-Step Guide

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In summary, the person is trying to find a calibration curve for an unknown solution's transmittance, and they are having trouble because the slope is negative and the y-intercept is high.
  • #1
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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  • #2
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
 
  • #3
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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  • #4
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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  • #5
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 :/.
 
  • #6
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
 
  • #7
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
 
  • #8
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
 

What is Beer's law?

Beer's law, also known as the Beer-Lambert law, is a principle that describes the relationship between the concentration of a solution and the amount of light it absorbs. It states that the absorbance of light by a solution is directly proportional to the concentration of the solution and the path length of the light through the solution.

Why is Beer's law important?

Beer's law is important because it allows scientists to quantitatively measure the concentration of a solution by measuring the amount of light it absorbs. This is useful in fields such as chemistry, biology, and environmental science where the concentration of a substance is often a critical factor.

What are the assumptions of Beer's law?

The assumptions of Beer's law include a monochromatic light source, a dilute solution, and a constant path length of light through the solution. Additionally, it assumes that there are no chemical or physical interactions between the solute and solvent that would affect the absorbance of light.

How is Beer's law used in real-world applications?

Beer's law is used in a variety of real-world applications, such as in spectrophotometry to measure the concentration of a substance in a solution. It is also used in environmental science to measure the concentration of pollutants in water samples and in the brewing industry to monitor the quality of beer during production.

What are the limitations of Beer's law?

Beer's law has limitations, such as only being applicable to dilute solutions and not accounting for complex interactions between solute and solvent. It also assumes that the solution is homogeneous and has a constant path length, which may not always be the case. Additionally, it is only applicable to certain wavelengths of light and may not accurately measure the concentration of solutions with highly concentrated or strongly absorbing substances.

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