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Topic: Beer's Law  (Read 3420 times)

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Offline StS

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Beer's Law
« on: January 31, 2012, 07:26:48 PM »
Individual spectra of pure compounds A and B were taken using 1.00 x 10 -5 M solutions of each, using a cell with a 5.00 cm path length.  The transmittance data for each solution at the two given wavenumbers is as follows:

Solution         % Transmittance at Wavenumber (cm -1)
                     
      -------------------------------------------------------------------------------------
Pure A               31.0           79.7
Pure B               97.4           20.0

An unknown mixture of A and B in a 5.00 cm cell gave transmittance values of 50.6% at 2022 cm -1 and 20.1% at 1993 cm -1.  Find the concentrations of both A and B in the mixture.

Using the initial data I can find the molar absorptivities for the pure compounds. But this is only for the wavenumber 79.7 and 20.0. When I try to find the concentration of A and B in the mixture I have two equations, and 4 unknowns. I don't have the molar absorptivities at the wavenumbers for the total mixture, nor the substances concentration's. Is there any way to convert the molar absorptivity value from one wavenumber to another?

Offline fledarmus

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Re: Beer's Law
« Reply #1 on: January 31, 2012, 08:36:17 PM »
I think there were a couple of headings left off of your table. It looks like the column reading 31.0 and 97.4 is supposed to be at 2022 cm-1 and the column reading 79.7 and 20.0 is supposed to be at 1993 cm-1. Like:

Solution         % Transmittance at Wavenumber (cm -1)
                        2022          1993
      -------------------------------------------------------------------------------------
Pure A               31.0           79.7
Pure B               97.4           20.0

Offline StS

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Re: Beer's Law
« Reply #2 on: January 31, 2012, 09:06:09 PM »
Ohhh, I see now. Thanks a lot!

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