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# Concentration/Photometry

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Using a spectrophotometer set at 400 nm, an absorbance reading of 0.050 was obtained for a solution of p-nitrophenol at pH 6.5. The molar extinction coefficient of p-nitrophenol 21500 M-1cm-1 and the pka for p-nitrophenol is 7. What is the total concentration of p-nitrophenol in the experimental solution.

https://brainmass.com/biology/biological-chemistry/concentration-photometry-167987

#### Solution Preview

Beers' law states that A = ecl.
Rearranging gives,
c = A/el

Solving gives,
c = 0.050/(21500 M-1 cm-1)(1 cm)
(Of course, we assume a 1 cm path length.)
c = 2.3x10-6 M

HOWEVER...

This is not the total concentration. This is the concentration of the protonated (neutral) form of p-nitrophenol, the species that absorbs at 400 nm and the one for ...

#### Solution Summary

Calculations with Beer's Law are shown and explained to find the total concentration of p-nitrophenol in an experimental solution.

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