CH271
Spectroscopy Basics
Work these problems in preparation of exam 3. This will not be collected and graded.
1. Calculate the frequency in hertz, the energy in joules, and give the region of the electromagnetic spectrum for electromagnetic radiation with a wavelength of 2.65 angstroms.
2. Calculate the frequency in hertz, the energy in joules, and give the region of the electromagnetic spectrum for electromagnetic radiation with a wavelength of 440 nm.
3. Calculate the absorbance for an analyte solution that has a percent transmittance of (a) 78.2% and (b) 15.0 %.
4. The transmittance (T) of a solution is found to be 35.0%. What is the percent transmittance (%T) of this solution if the volume is doubled? (Hint: What happens to the concentration when this occurs?)
5. Calculate the percent transmittance (%T), absorptivity (a in cm-1 ppm-1), molar concentration (M), and ppm for an analyte solution with an absorbance (A) of 0.387 when measured in a 2.00 mm cell (b), and a molar absorptivity (e) of 3.77 x 103 M-1 cm-1. Assume a formula weight of 250.0 g/mol whenever necessary.
6. Calculate the absorbance, pathlength (cm), molar absorptivity (e in M-1 cm -1), and ppm for a 1.35 x 10-4 M analyte solution with a percent transmittance (%T) of 44.9, and an absorptivity (a) of 0.0258 cm-1ppm-1. Assume a formula weight of 250.0 g/mol whenever necessary.
7. The concentration of phenol in a water sample is determined by separating the phenol from nonvolatile impurities by steam distillation, followed by reaction with 4-aminoantipyrine and K3Fe(CN)6 at pH 7.9 to form a colored antipyrine dye. A phenol standard with a concentration of 4.00 ppm has an absorbance of 0.424 at a wavelength of 460 nm using a 1.00 cm cell. A water sample is steam-distilled, and a 50.00 mL aliquot of distillate is placed in a 100 mL volumetric flask and diluted to volume with distilled water. The absorbance of this solution is found to be 0.394 using a 1.00 cm cell. What is the concentration of phenol in ppm in the water sample in the flask?
8. Using the following data, calculate the concentration of analyte R in an unknown solution.
Sample 5.00 ppm Standard Final
Volume (mL) Volume (mL) Volume (mL) Absorbance
1 15.00 0 50.0 0.235
2 15.00 5.00 50.0 0.509
9. State the difference between single and double beam spectrophotometers and explain how each measures the transmittance of a sample. What source of error in a single beam instrument is absent in the double beam instrument?
10. What is an isosbestic point? When are isosbestic points observed?
11. Why does a compound which has a visible absorption maximum at 480 nm appear to be red?
12. What color would you expect to observe for a solution with a visible absorbance maximum at 580 nm?
13. Draw the expected spectrophotometric titration curve for each of the titrations under the conditions indicated. The reaction is: A + T ® P where A is the analyte, T is the titrant, and P is the product.
a. eA = 0, eT = 0, eP > 0
b. eA = 0, eT > eP > 0
14. Why is it important to make sure that cuvets used during spectral measurements are free from smudges and fingerprints?
15. Why is it important to make absorbance measurements for an analyte at the wavelength of maximum absorbance, i.e. a peak in the spectrum?
16. Using the 0.020 mg/mL standard solution of Fe2+ as directed in the experiment, a student got the following calibration curve data.
mLs of standard Concentration
used - diluted to 100.0 mL (mg/mL) Absorbance
0.00 0.0 0.0
5.00 1.0 x 10-3 0.36
10.00 2.0 x 10-3 0.80
15.00 3.0 x 10-3 1.16
20.00 4.0 x 10-3 1.62
25.00 5.0 x 10-3 1.96
Make a graph of absorbance vs. concentration of Fe (mg/mL). The student dissolved 300.0 mg of his sample in 500.0 mLs. He/she took 3 different volumes of the sample solution, added the appropriate reagents to a 100.0 mL volumetric flask (as was done with all the standards) and got the following data:
mLs of unknown Absorbance
10.00 1.14
8.00 0.90
6.00 0.66
Calculate the average % Fe in the sample plus the standard deviation and ppt error for the data.
Answers will depend somewhat on method of data treatment
Data obtained from y = mx + b using computer Excel program.
m = slope = 398, b = -0.01
% Fe = 4.76 ± 0.07 with 15 ppt
17. Analyte A and analyte B form colored complexes when treated with a certain reagent C in 1 M H2SO4(aq). Species A has an absorption maximum at 415 nm, and the species B has an absorption maximum at 513 nm. A 0.01000 M solution of species A exhibits an absorbance of 0.805 at 415 nm and 0 at 513 nm, while a 0.0100 M solution of the species B exhibits absorbances of 0.220 and 1.200 at 415 and 513 nm, respectively. A 1.000 g unknown sample containing both A and B was dissolved, treated with reagent C, and diluted to a final volume of 100.0 mL in a volumetric flask. The absorbance of the solution was 0.508 at 415 nm and 0.600 at 513 nm. What are the percentages of A and B in the mixture?