Week 4: The Spectra of Conjugated Dyes and Investigation of Beer's Law Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Prelab Problems 1. If bright red Koolaid is colored by a single dye, with a single, broad absorption peak, predict the wavelength range of maximum absorbance for this Koolaid solution. Use the Spectral Colors applet on the ChemLab website to create a spectrum of a bright red solution, with a background color comparable to red Koolaid. Sketch a possible absorption spectrum and transmission spectrum you obtain, labeling the wavelengths of maximum absorbance and maximum transmission. 2. Consider an electron confined to a potential box. On graph paper, draw an accurate energy level diagram showing the spacing of the energy levels for the first five energy levels, in units of h2/8m2. For Dye A, count the number of free electrons. Use your diagram to draw an accurate energy level diagram for the ground and first excited electronic states of Dye A. Use the web applet, Electrons-in-a-box, to estimate the energy of the highest occupied energy level and calculate the energy difference between the ground and first excited states. Record the energy difference and wavelength of light required to excite an electron from the ground to excited state. Use the correct number of electrons for Dye A, the approximate box length, estimated from the bond lengths, and the mass of an electron. Repeat the applet calculations for Dye B. 3. The following concentration vs. absorbance data were collected for Dye A. Plot a Beer's Law plot of this data and estimate the extinction coefficient, given that the path length of the sample was 1.17 cm. Concentration Absorbance 1.0 x 10-4 M 0.580 8.0 x 10-5 M 0.480 6.0 x 10-5 M 0.350 4.0 x 10-5 M 0.239 2.0 x 10-5 M 0.150 0 0 (this is the absorbance of the blank) A solution of dye A with an unknown concentration was found to have an absorbance of 0.325. What is the concentration of Dye A in the solution? 4. The directions for making a particular flavor of Koolaid say to add the 3.9 g of drink mix in the package to 2 quarts of water. Assuming the final volume of the Koolaid solution is 2.0 quarts, calculate the concentration of the Koolaid, in g mix per L of solution. Calculate the mass of Koolaid needed to make 25.0 mL of solution, at this concentration. Describe the technique you will use to make up a Koolaid solution using a 25 mL volumetric flask. Consult the Volumetric Flask page on the ChemLab website, for information about this kind of flask and the correct way to use it.
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