1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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

    Borek

    User Avatar

    Staff: Mentor

    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

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Beers Law, maximum molar absorptivity of carbonyl molecule
  1. Beer's law (Replies: 0)

  2. Beers Law? (Replies: 12)

  3. Beer's Law (Replies: 1)

  4. Beer's law (Replies: 7)

Loading...