Chem 2113 Test 4 Spring 1998

Questions are worth 25 points each. OMIT ONE QUESTION by clearly writing OMIT in the space provided for your work. If you fail to mark OMIT on a question I will omit the last question of the test. Show your work and circle your answers for full credit.

1. Define the following terms. You may use equations where appropriate; however, be sure to define all symbols in any equation you use.

Chromophore

Quantum yield

Quenching

Stokes shift

Transmittance

2. Perform the following conversions:

a) A frequency of 122.9 MHz to wavelength.

b) The energy of a photon with a wavelength of 24.1 µm.

c) 41.5%T to A

d) An einstein of light is equivalent to 1 mol of photons for a specified wavelength of light. Calculate the energy in kJ for an einstein of light of 405 nm. (1 mol = 6.022 x 10²³)

3. A sample of copper is to be determined spectrophotometrically using the method of standard additions. A 0.0741 g sample of a copper ore is crushed, and the resultant ore flour is digested in 3M HNO₃. The stock solution is then prepared by dilution of the filtrate to 500.0 mL in a volumetric flask. The analysis then proceeds as follows:

mL Stock; mL 10.0 ppm Cu std. ;Final volume; %T

10.0; 0.00; 100.0; 63.4

10.0; 5.00; 100.0; 31.8

Calculate the percent copper in the ore sample.

4. Consider the simultaneous determination of two components. Species A absorbs strongly (molar absorptivity = 12,500) at 347 nm and exhibits a weaker absorption (molar absorptivity = 524) at 515 nm. Species B absorbs strongly (molar absorptivity = 9550) at 515 nm and does not absorb at 347 nm. A mixture of A and B was determined to have an absorbance of 0.642 at 347 nm, and an absorbance of 0.755 at 515 nm. What is the concentration of species A and B in this mixture. All measurements were made in a 1.50 cm cell.

5. Consider the fluorimetric determination of aluminum with morin. Standards containing aluminum were prepared, and the following data were collected:

Al (ppm) ; Fluorescence

1.00 ; 5.17

2.00 ; 8.23

5.00 ; 20.0

7.50 ; 26.8

Calculate the slope and the y-intercept for the linearly related data.

The unknown was prepared by dissolving a 0.100g sample containing aluminum and diluting this sample to 200.0 mL. A 1.00 mL aliquot of this solution was treated with morin and diluted to 50.00 mL. This solution had a fluorescence of 16.2. Calculate the percent aluminum in the unknown.

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