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Beers Law, maximum molar absorptivity of carbonyl molecule

  1. Apr 1, 2012 #1
    "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?
  2. jcsd
  3. Apr 1, 2012 #2


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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...
  4. Apr 1, 2012 #3
    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.
  5. Apr 2, 2012 #4


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    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.
  6. Apr 4, 2012 #5


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    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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