ChemLab - Chemistry 6 - Spectra of Conjugated Dyes & Beer's Law

Spectra of Conjugated Dyes & Beer's Law

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Question 1

Why doesn't the wavelength that Dye A and B absorb at, according to the spectrometer, agree with the wavelength that the applet gives me? Is there something wrong with the spectrometer?

Question 2

Should I include the origin in my least squares fit of my Beer's Law data?

Question 3

How do I calculate the percent by mass of the dye in the Koolaid?

Question 4

I am flummoxed by the uncertainty analysis on the food dye concentration for the following reason: The lab manual asks for "uncertainty with which you measured the absorbance, estimated in the lab." My calculations are all ready to rumble but I don't know what this value for uncertainty of the spec 20 is.

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Questions & Answers

Question 1

Why doesn't the wavelength that Dye A and B absorb at, according to the spectrometer, agree with the wavelength that the applet gives me? Is there something wrong with the spectrometer?

Professor Milde

Lack of agreement between your experiment and the theoretical model can be the result of problems with either the experiment or the model. If the wavelength of maximum absorbance measured by the spectrometer matches your prediction based on the color of the solution, there is probably not a problem with the spectrometer. This would implicate the theory as the source of disagreement. Are the assumptions of the theory exact or approximate? Is it realistic to expect precise agreement between experiment and this particular theory?

Question 2

Should I include the origin in my least squares fit of my Beer's Law data?

Professor Milde

You measured the absorbance of a solution with zero concentration and zero absorbance--the blank--so yes, you should include a point at the origin in your Beer's Law data.

Question 3

How do I calculate the percent by mass of the dye in the Koolaid?

Professor Milde

In the lab manual, the concentrations asked for are the molarity of the Koolaid solution, the moles dye per gram drink mix, and % dye by mass in the powdered drink mix. To determine the first concentration listed, you should calculate the molarity of the dye in the Koolaid solution from your Beer's law plot. This means the plot should be A vs. concentration in units of MOLARITY. Next, from the molarity and the volume of the Koolaid solution, you can determine the moles of dye in the mass of drink mix used to make up your solution. This allows you to calculate the second result, the moles of dye per gram mix. Finally, you can convert this to grams dye per gram mix, using the molecular mass of the dye. This can be multiplied by 100 to calculate the % dye by mass in the dry mix. The molar masses of all food dye possibilities are given in the lab manual with the dye structures.

Question 4

Professor Milde

If you measured the absorbance value of your Koolaid sample as 0.34, was it 0.340 or 0.34? How accurately could you read the meter? ± 0.001? ± 0.005? ± 0.01? This is the uncertainty value for Spec 20 readings that you need to complete your analysis. You should estimate this value in the lab, when you make the measurments. If you have already finished the experiment and are working on your write-up, can you estimate the uncertainty from the values your wrote in your notebook?

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