- Absorption Spectrum Analysis (2p). Below is the absorbance versus wavelength plot (absorption spectrum) for a stock FD&C food dye. Clearly state the λmax.
- Dilution Calculations (10p). You wish to construct an absorbance versus concentration (A vs. C) plot for the FD&C food dye of Question 1 above. Suppose the stock dye solution is 9.65 x 10-6 M. Complete the following table (fill in the blanks) showing one example of each type of calculation performed. (Remember to properly present all calculations following the format introduced in Lab Assignment 1: first state symbolically (with mathematical symbols, not actual values) the equations, second, present the calculations in which actual values are plugged in and the results shown. Include units in the calculations and control significant figures in the final answer.)
Table LA3-2 Parallel Dilution Set Preparation
Stock Concentration Volume of Stock (mL) Volume of Diluent (mL) Diluted Sample Concentration (M)
9.65 x 10-6 M 5.00 0.00
3.67 7.08 x 10-6
2.50 2.50 4.83 x 10-6
1.33 3.67
0.33 6.37 x 10-7
Table LA3-3 A vs. C: Determination of ε for a Food Dye
Absorbance Concentration (M)
0.803 2.24 x 10-5
0.577 1.64 x 10-5
0.403 1.12 x 10-5
0.223 5.97 x 10-6
0.053 1.49 x 10-6
- Determining a Molar Absorptivity (8p). Assume you wish to determine the molar absorptivity (ε) for another (different) food dye. You take a 2.24 x 10-5 M stock solution of this dye and determine the λmax from an absorption spectrum of this stock. Then you prepare a dilution set and read the absorbance at the dye’s λmax. This is the data presented below. Using only the data below, find ε for this dye. Properly title the plot and label the axes correctly. Include the R2 value and the linear trend-line equation. You may need to adjust the sig figs for the R2 value and linear trend-line equation in EXCEL. Properly scale the plot if necessary. Paste in the complete plot below. You are expected to remember this format for all subsequent lab work without being prompted again. State your ε value. Use correct sig figs and units.
- Determining the Dye Concentration (5p). Using the results of Question 3 and assuming b = 1.00 cm, if a sample gives an absorbance of 0.578, what is the concentration of the dye in the sample? (Remember to properly present all calculations following the format introduced in Lab Assignment 1: first state symbolically (with mathematical symbols, not actual values) the equations, second, present the calculations in which actual values are plugged in and the results shown. Include units in the calculations and control significant figures in the final answer.)