CHM 411/511
Midterm Exam
March 1, 2007
Name
Part I. Short answer. Do any 7 of the following 8 questions. 6 points each
1.
Circle all of the following statements that are true about UV-VIS absorption of a molecular liquid.
a)
b)
c)
d)
e)
2.
A line absorption spectrum is produced
The σ→σ* transitions are likely to be in the visible region of the spectrum.
The π→π* transitions require less energy than σ→σ* transitions.
The n→π* transitions are at a longer wavelength than the σ→σ* transitions.
Absorption bands with a large molar absorptivity imply a long lifetime for the electronically
excited state.
Explain all of the relaxation processes that:
a) an atom in an excited state may undergo.
b) a molecule in an excited state may undergo.
3.
For which of the following transitions would you expect multiple wavelengths because of p orbital
splitting? Briefly explain. (6 pts)
Mg atom 3p (singlet excited state) to 3s
Mg atom 3p (triplet excited state) to 3s
Na atom 3p to 3s
4.
For the molecules shown below:
Tell which molecule is expected to show the largest fluorescence quantum yield, and briefly explain
why.
5.
Draw a block diagram of a fluorescence spectrometer that is based on a tungsten light bulb.
6.
If you had a spectrum of a sample and it consisted of discrete, sharp lines, what could
you conclude about the identity of the sample?
7.
What is destructive interference of radiation?
8.
What method of analysis would you use for quantitative measurements for:
a) determining the amount of Ca2+ in milk
b) determining the amount of an aromatic compound
Part II. Calculations. You must show all of your work to receive full credit.
1.
A bandpass filter has a dielectric layer with a thickness of 85.0 nm and a dielectric constant of 1.75
Calculate the first order wavelength transmitted by the filter. (3 pts)
2.
Calculate the percent loss of intensity due to reflection of a perpendicular beam of red light as it passes
through a quartz plate. (Assume that for red radiation the refractive index of quartz is 1.64 and air is
1.00). (5 pts)
3.
In high temperature sources, sodium atoms emit a doublet with an average wavelength of 1139 nm. The
transitions arise from the 4s to 3p (for which there are two closely spaced) state. The ratio of
degeneracies for these transitions is therefore ½. Calculate the ratio of the number of excited state atoms
in the 4s state to the number of ground state atoms in the 3p in an air/acetylene flame operating at
2200oC. (5 pts)
4.
Ti4+ forms complexes with H2O2 that absorb in the visible portion of the electromagnetic spectrum. A
sample containing Ti4+ was treated with excess H2O2 and the UV-Vis spectrum was measured. The UV-Vis
spectrum was also obtained for a standard solution of Ti4+ treated with excess H2O2. The absorbance values
measured at 410 nm are summarized in the table below. Calculate the concentration of Ti4+ in the unknown
solution. (8 pts)
[Ti4+]
Absorbance at 410 nm
1.32 mM
0.96
Unknown
0.74
Part II.
1.
Describe the advantage of a four-level laser system over a three-level type.
2.
Explain fluorescence and phosphorescence in terms of an energy state diagram (draw one and explain
it). How do you distinguish fluorescence and phosphorescence in terms of wavelengths and lifetimes?
What could be done to enhance the phosphorescence signal from a molecule?
3.
Give the main components of a typical optical spectroscopic instrument and give the ideal characteristics
for each.
4.
What are calibration curves? What is standard addition? Discuss how calibration curves are used
to define the sensitivity and dynamic range of a method. What are matrix effects? If matrix
effects are severe, which method is better - calibration curve or standard addition? (you may want to
draw a calibration curve to help answer the question).
5.
Compare and contrast the sources of radiation between molecular absorbance,
fluorescence and atomic absorbance instruments.