You want to determine the concentration of a dye in an unknown solution using vi

Question

You want to determine the concentration of a dye in an unknown solution using visible spectroscopy. The dye is known to exhibit maximum absorbance at 573 nm, with a molar absorptivity () of 84,300 L/(mol cm). The original solution is too concentrated to give an absorbance reading below 1.00 at , so it is diluted by transferring 10.00 mL to a 25,00 mL volumetric flask and diluting to the mark. The dilute solution then exhibits an absorbance of 0.748 atAmax. What is the concentration of the dye in the original solution? 0

Explanation / Answer

Molar absorptivity at measured wavelength = 84300 M-1s-1

Assuming path length = 1cm

absorbance = 0.748

according Beer-Lamberts law

Absorbance = extinction cofficient x concentration x path length

Thus

concentration = 0.748/ 84300 = 8.87 x 10-6 M

This is the concentration after dilution.

For dilution 10 mL is dilute to 25 mL

Dilution factor = 2.5

Actual concentration = 8.87 x 10-6 x 25 = 2.218 x 10-4 M

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