Anal Chem 1 In-class Spectrophotometry Problems Fall 2002

1. Anal Chem 1 In-class Spectrophotometry Problems Fall 2002

2. 1 - Calculate the frequency (Hz), wavenumber (cm-1) and energy (kJ/photon and kJ/mol photons) of a) ultraviolet light with a wavelength of 250 nm and b) infrared light with a wavelength of 2.50 m

3. 2 - Convert the following transmittance (T) values to absorbances (A)

4. 3 – The absorbance of a 2.31 x 10-5 M solution is 0.822 at a wavelength of 266 nm in a 1.00 cm-cell. Calculate the molar absorptivity at 266 nm.

5. 4 - A 15.0-mg sample of a compound with a formula mass of 384.63 was dissolved in a 5-mL volumetric flask. A 1.00 mL aliquot was withdrawn, placed in a 10-mL volumetric flask and diluted to the mark. (a) Find the concentration of the sample in the 5-mL flask. (b) Find the concentration of the sample in the 10-mL flask. (c) the 10-mL sample was placed in a 0.500-cm cuvet and gave an absorbance of 0.634 at 495nm. Find the molar absorptivity at 495nm

6. 5 - A compound with a formula mass of 292.16 was dissolved in a 5-mL volumetric flask. A 1.00-mL aliquot was withdran, placed in a 10-mL volumetric flask, and diluted to the mark. The absorbance measured at 340 nm was 0.427 in a 1.000-cm cuvet. The molar absorptivity for this compound at 340nm is  = 6130 M -1 cm -1. (a) Calculate the concentration of compound in the cuvet. (b) What was the concentration of compound in the 5-mL flask? (c) How many milligrams of compound were used to make the 5-mL solution?

7. 6 -Two blue solutions, each known to contain only one absorbing species, had the following absorbances in a 1.00-cm cuvet. Do the two solutions contain the same substance? Explain.

8. 7 - A 10.00-mL sample containing benzene was diluted to 250.0 mL with hexane and the resulting solution had an absorbance of 0.364 at 204 nm in a 5.00-cm cuvet.  for benzene at 204 nm has a value of 7900. Calculate the concentration of benzene (mg/mL) in the original solution.

9. 8 - A 200-mL sample of well water was treated with excess hydroxylamine hydrochloride to reduce all iron to Fe+2. After the addition of acetic acid/sodium acetate buffer to adjust the pH, an excess of 4,7-diphenyl-1,10-phenanthroline was added and the mixture diluted with water to 250 mL in a volumetric flask. This solution had a transmittance of 0.631 at the max of 533nm. A second solution, prepared by dissolving 72.5 mg of pure iron wire in acid was treated similarly except the final dilution was to 1.00 liter. A 10.00-mL aliquot was diluted in 100.0mL volumetric flask had a transmittance of 0.515 using the same cell as used in the first measurement. Calculate the ppm of iron in the original sample of well water.

10. 9 - Soluble phosphate is often analyzed by reacting is with a solution of sodium molybdate (Na2MoO4) and hydrazine sulfate. An intense blue color forms which is the basis of the spectrophotometric anlaysis for phosphate. max at 830nm with  = 24200 M-1 cm-1. Because samples of phosphate that are of interest often have high concentrations of other species (matrix effect) the method of standard addition is generally used. 100-mL sample of natural water was treated as described above with the molybdate and hydrazine reagents and diluted in a 250-mL volumetric flask. Its absorbance was measured at 830 nm using a 1.00-cm cuvet and found to be 0.290. A second portion of 100-mL of the sample was treated identical to above except 10.00mL of a standard phosphate solution whose concentration is 1.0 x 10-4 M PO4-3 is added before the two reagents and dilution to 250 mL. Its absorbance at 830 nm was found to be 0.585 using the same 1.00-cm cuvet. What is the original concentration of phosphate in the natural water sample?