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Unformatted text preview: nse the cuvette carefully with distilled water between each run. Put
your first standard solution in the cuvette, and place the cuvette into the colorimeter.
Type the concentration in, and when you are sure you are ready, click the “read” button.
As soon as you have, the system will read the absorbance, and record both the
concentration and absorbance which should appear in your table. You are then prompted
for the second standard; repeat this procedure, starting with rinsing the cuvette, for each
standard. When all standards are finished, have the Pasco system read the absorbance for
each unknown you are given.
Be sure to print out the data when you are finished. We will discuss certain
procedures in lab (such as considerations in rinsing cuvettes), and how to manipulate
your data as well. Be sure to keep an accurate lab notebook as you perform this
procedure. A list of questions you will want to discuss as part of the report will be
covered when we discuss how to write a lab report. Dakota State University page 207 of 232 Experiment 20: Beer’s Law General Chemistry I and II Lab Manual HyperChem component:
Azobenzene (an acaricide: kills parasites such as ticks and mites) has the
following structure: Using the build tool, build this molecule in HyperChem. Be careful to get the correct
double bonds. Remember that with the “add H and model build” tool, you need not put
hydrogens in explicitly.
Choose “Setup…Semiempirical.” Choose “AM1” an click “OK.” Go to
“Compute…Geometry Optimization” and click OK to keep the default values. Allow the
system to find the optimal geometry for the AM1 basis set.
Next choose “Computer…Vibrations” and allow HyperChem to complete the
calculations. Finally, go to “Compute…Vibrational Spectrum.” This gives you the
calculated values where peaks should be found. Clicking on any one of these lines will
show you the wavenumber of that line, its intensity and frequency. Clicking “OK” will
show the particular vibrational mode of the molecule that gives rise to this particular
color of light being absorbed if “animate vibrations” is selected. This shows how light
energy is absorbed by molecules; by exciting vibrational modes of the molecule.
How does this spectrum compare to the true spectrum of azobenzene? What color
do you suppose it is? Calculations:
Generate a calibration curve for the standards. From the light absorbed and the
calibration curve, determine the concentration of the unknown solution. Dakota State University page 208 of 232 Experiment 20: Beer’s Law General Chemistry I and II Lab Manual Observations:
Sample Concentration Absorbance Observations: Dakota State University page 209 of 232 Experiment 20: Beer’s Law General Chemistry I and II Lab Manual Pre-Lab Questions:
1. Chemists much prefer to measure absorbance over percent transmittance. Why?
2. Explain Beer’s law in your own words. Dakota State University page 210 of 232 Experiment 20: Beer’s Law General Chemistry I and II Lab Manual Post-Lab Questions:
1. The story comes down of a graduate student d...
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