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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: palomino96 on January 15, 2007, 03:42:42 AM

Title: Beer's Law A=ebc
Post by: palomino96 on January 15, 2007, 03:42:42 AM

I need help with finding out what e is?  I know that it is in L mol-1 cm-1 and the wavelength given is calculated somehow in the equation.  Can someone please help?

Shawna ???

Title: Re: Beer's Law A=ebc
Post by: chiralic on January 15, 2007, 05:45:58 AM

molar absorptivity coefficient, in older literature it is sometimes called the extinction coefficient

Chiralic

Title: Re: Beer's Law A=ebc
Post by: palomino96 on January 15, 2007, 02:45:45 PM

Chiralic,

I know what it is called.  I have did all kind of research and searches and cannot find out what to input for e.  An example in my lab book was "The molar absorptivity of cuproine at wavelength=546nm is 6220 L mol-1 cm-1.  I cannot figure out how to break the 546nm down to get the 6220 L mol-1 cm-1.  Do you know how?

Shawna ???

Title: Re: Beer's Law A=ebc
Post by: enahs on January 15, 2007, 03:03:36 PM

You measure it analytically.

Title: Re: Beer's Law A=ebc
Post by: Yggdrasil on January 15, 2007, 03:19:36 PM

e is different for each substance.  You either need to measure a set of standard solutions of known concentration to determine it empirically or look it up in the literature.

Title: Re: Beer's Law A=ebc
Post by: palomino96 on January 15, 2007, 04:27:01 PM

I am using 2.00 mL of .1 M CuSO4.  The %T=86.8, A=0.06, wavelength=700nm.  The example they are giving in my lab manual is to use formula c=A/eb.  In my case would be c=0.06/e times 1.0cm.  I feel really stupid because I know it is something that is going to seem so simple once I know how to calculate e.  Any help is really appreciated.  This is only my second chemistry class so I don't know a whole lot.

Thanks,
Shawna

Title: Re: Beer's Law A=ebc
Post by: chiralic on January 15, 2007, 05:03:52 PM

As wrote Yggdrasil:

Quote

You need to measure a set of standard solutions of known concentration

How? Clue: Beer's Law Plot (A vs C), check the slope (Hint: A=ebC). Also you know the
diameter of the cuvette that you are using and then you'll get "e,molar absorptivity coefficient"

Regards,

Chiralic

Title: Re: Beer's Law A=ebc
Post by: chiralic on January 15, 2007, 05:27:06 PM

More hint:
http://www.dartmouth.edu/~chemlab/chem6/cobalt2/full_text/write-up.html

Title: Re: Beer's Law A=ebc
Post by: chiralic on January 15, 2007, 05:31:08 PM

Help on line...about your post:

http://www.chm.davidson.edu/ChemistryApplets/spectrophotometry/BeersLaw.html

Title: Re: Beer's Law A=ebc
Post by: enahs on January 15, 2007, 08:03:39 PM

Quote from: palomino96 on January 15, 2007, 04:27:01 PM

I am using 2.00 mL of .1 M CuSO4.  The %T=86.8, A=0.06, wavelength=700nm.  The example they are giving in my lab manual is to use formula c=A/eb.  In my case would be c=0.06/e times 1.0cm.  I feel really stupid because I know it is something that is going to seem so simple once I know how to calculate e.  Any help is really appreciated.  This is only my second chemistry class so I don't know a whole lot.

Thanks,
Shawna

Ok, wait.

In the equation
A = ?bc
Or as you wrote it, c = A/?b
But you know A, c and b, so solve for ?
? = A/cb

You know the concentration, just plug it in and solve for ?.

Title: Re: Beer's Law A=ebc
Post by: palomino96 on January 15, 2007, 08:26:23 PM

So this is what I would get.
e=.06/(.1Mx1.0cm)
e=.6 mol/L cm