Allura Red (FD&C; Red 40, molar mass = 496.42 g/mol) is a common red dye that is
ID: 530225 • Letter: A
Question
Allura Red (FD&C; Red 40, molar mass = 496.42 g/mol) is a common red dye that is used in cosmetics, medicine, and food. A student made three different solutions of red dye and used them to construct a Beer's law calibration curve using the wavelength of maximum absorption. The path length of the cuvette used was 1.20 cm The equation for the line of best fit is given on the graph. A student wanted to determine the concentration of Allura red dye in a 1.00 L bottle of red Mountain Dew. The student used a 10-mL volumetric pipet to transfer 20 mL of the Mountain Dew to a 100-mL volumetric flask and then added distilled water to the mark on the flask. The absorbance of this diluted solution at 504 nm was 0.480. Using the line of best fit given above, determine the concentration (in ppm) of Allura red dye in a 1.00 L bottle of red Mountain Dew. You can assume the density of Mountain Dew is 1.00 g/mL. Was it necessary for the student to dilute the red Mountain Dew before taking the absorbance? Briefly explain your reasoning. How do you expect the slope of the line-of-best fit for a Beer's law calibration graph of blue Mountain Dew at 504 nm to compare to the slope of the line-of-best fit for a Beer's law calibration graph of blue Mountain Dew at 620 nm? Briefly explain your answer.Explanation / Answer
From the calibration plot
1. Absorbance of allura red dye solution = 0.480
0.480 = 30451x + 0.0132
So,
concentration of diluted solution = (x) = 1.53 x 10^-5 M
2. Beer Lambert's law is satisfied only in dilute solutions. At very high or very low concentrations, the deviation from the law is seen as either all of the light passing through the sample is absorbed or almost none is absorbed. Therefore, it was necessary to dilute the concentrated sample of allura red prior to measurement.
3. The slope expected for the mountain dew at 504 nm would be higher than the slope seen at 620 nm as the lambda max for allura red is 504 nm. So at other wavelengths it would not show maximum absorbance.
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