Beers Law, maximum molar absorptivity of carbonyl molecule

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Discussion Overview

The discussion revolves around the application of Beer's Law to determine the path length of a sample containing a carbonyl group that results in specific reductions in the intensity of transmitted radiation. Participants explore the implications of the maximum molar absorptivity value and the relationship between absorbance, path length, and concentration.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about the lack of concentration information needed to solve for path length using Beer's Law.
  • Another participant suggests substituting different path lengths into the Beer's Law equation to explore how absorbance changes with varying path lengths.
  • A different participant reiterates the challenge of having two unknowns (path length and concentration) while trying to solve for one variable.
  • One participant questions the interpretation of "initial intensity," proposing that it may refer to the result of a previous experiment rather than the source intensity.
  • There is a discussion about how to achieve specific reductions in intensity by adjusting the path length, with a focus on the implications of these changes.

Areas of Agreement / Disagreement

Participants generally express uncertainty about how to proceed with the problem due to the missing concentration value and differing interpretations of "initial intensity." Multiple competing views on how to approach the problem remain unresolved.

Contextual Notes

Participants note the dependence on the concentration value, which is not provided, and the ambiguity surrounding the definition of "initial intensity." These factors contribute to the complexity of the problem.

Who May Find This Useful

This discussion may be useful for students studying Beer's Law, particularly those grappling with the application of the law in scenarios involving unknown concentrations and path lengths.

Puchinita5
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"Given that the maximum molar absorption coefficient of a molecule containing a carbonyl group is 30 dm3 mol-1 cm-1 near 280 nm, calculate the thickness of a sample that will result in (a) half the initial intensity of radiation, (b) one-tenth the initial intensity."

My attempt:

What is confusing me is I feel like I need the concentration to be given, but it's not.

A = -log(T) = εbc
where T is transmittance, ε is molar absorptivity, b is path length, and c is concentration.

I am understanding that the problem is looking for the path length, but I'm not completely sure about this.

I would do this for part A.

-log(0.5) = (30)*b*c
and solve for b.


how can i solve this? what am i missing?
 
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What you need to understand is there is a relationship of path length (b) to absorption (A). Substitute 2b (twice the path length, right?) into your Beer's Law equation and see how that affects the absorbance. Now try 1/2b and 1/10b...
 
Well yes, but I don't see how that helps me since I am still not given a concentration. I understand the general mathematical relationship of what happens, but I still feel like I am told to solve for one variable, but I have two unknowns. ? Perhaps I am not understanding what the problem is asking me.Obviously, if you increase the path length, you increase the absorbance. But not sure why this helps.
 
If you are missing something, I am missing it as well. No problem with calculating bc, but I don't see how you can calculate just b.
 
What do you think 'initial intensity' means? Do you think it means the intensity of the source or the result of the previous experiment? Only way this question can be answered is if you take it to mean the result of a previous experiment.

A sample of the carbonyl compound is measured in a cuvette of path length 'b'. The initial intensity of the passed radiation is measured by the photomultiplier. How do you reduce the initial intensity by changing path length so that the intensity of passed radiation is half as great? One tenth as great?
 

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