Sonia B.

asked • 09/30/17

Molar abosrptivity

Calculate the molar absorptivity
1. Blue Dye (concentration 40mg/L) (Peak/ λmax is 630)
2.  Red Dye (concentration 60mg/L) (Peak/ λmax is 520)
2. Red Dye (concentration 70mg/L) (Peak/ λmax is 500)
 
 

 

1 Expert Answer

By:

Sonia B.

Blue: absorbance 0.7
Red absorbance: 2.3
Red absorbance  2.1
Report

09/30/17

J.R. S.

tutor
With this additional information, you can calculate a "pseudo" molar extinction coefficient.  It isn't a "real" molar absorbtivity, because you are using units of mg/ml as concentration instead of using moles/liter (M). If you know the molar mass (molecular weight) of the dyes, you can convert to M and get a real molar absorbtivity. Also, the λmax is not involved in calculating molar absorbtivity.
Blue λmax is 630:  0.7 = e x 40 mg/L x 1 cm and e = 0.7/40 = 0.0175
Red λmax is 520: 2.3/60 = 0.038
Red λmax is 500: 2.1/70 = 0.03 
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10/01/17

Sonia B.

Can you conclusively tell the two red dyes apart using this technique? Explain how the differences in λ
max between the red and the blue dyes relate
to the different colours of the dyes?
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10/29/17

J.R. S.

tutor
I would say that you CANNOT conclusively tell the red dyes apart.  Their λmax values are too close, and they have essentially the same extinction (0.038 v. 0.03).  
 
The color of the dyes (red v. blue) is related to their chemical structure and this in turn affects how light is absorbed.  When white light passes through a colored substance, a certain portion of the mixed wavelengths is absorbed. The remaining light will then assume the complementary color to the wavelength(s) absorbed.  So, if 500 λ wavelength is absorbed, the color will be predominantly red and if 600 λ wavelength is absorbed the color is predominantly blue. 
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10/29/17

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