Chem 220 - Exam IV – Spring 2010
Name
Date
SHOW ALL WORK!!!!!!
GOOD LUCK 
Points Earned
I.
II.
III.
IV.
BONUS
Total Points
I. Multiple Choice (15 points)
1. Absorption of IR radiation involves
a. rotational energy levels
b. vibrational energy levels
c. electronic energy levels
2. Fluorescence involves the excitation and emission of light based on the theory that
a. the emission wavelength and excitation wavelength are the same
b. the emission wavelength is shorter than the excitation wavelength
c. the emission wavelength is longer than the excitation wavelength
3. FRET stands for
a. Fluorescence Recovery Electron Transfer
b. Fluorescence Resonance Energy Transfer
c. Fluorescence Recovery Energy Transfer
4. If the refractive index of a medium is lower then air, then the speed of light through that
medium will
a. be lower
b. be higher
c. stay the same
5. The correct order of increasing frequency of the following types of electromagnetic
radiation is
a. Radio < UV < visible < x-ray
b. Radio < visible < UV < x-ray
c. X-ray < UV < visible < radio
II. True or False (If False, Explain why (9 pts total))
6. In UV/Vis spectroscopy, n to π* transitions occur at lower wavelengths than π to π*
transitions
7. The excitation wavelength is blue shifted from the emission wavelength in the process of
fluorescence
8. FRAP stands for Fluorescence Resonance after Photobleaching.
III. Solve (41 pts TOTAL)
9. (10 pts) Calculate the molar absorptivity of a 2.34 ppm solution of caffeine (194 g/mol)
that was measured in a 2.00cm cuvette to have an absorbance of 0.50 at a wavelength of
600nm.
10. (16 pts TOTAL)
a. (10 pts) How much more energy would be required to excite a benzene molecule
which absorbs energy at a wavelength of 254 nm than a CO molecule which absorbs energy of a
wavelength of 1600 cm-1.
b. (6 pts) Explain, in terms of the molecular structures of benzene and CO, why benzene
requires more energy to excite it then CO.
11. (15 pts) Use the following information that was used to make a Beer’s Law Plot to
determine the wt% of Cr in your original solution of 0.100L if a diluted solution of 10.00L
measures an absorbance of 0.09.
Cr Standards
1
2
3
4
Concentration
(ppm)
1
6
15
34
%
Transmittance
99
87
70
45
IV. Short Answer (35 pts TOTAL)
12.(6 pts) Draw the following transitions
a. Fluorescence
b. IR
c. UV
E2
E1
E0
d. Visible
13. (4 pts) Explain two processes that reduce fluorescence intensity and how you would correct
for them.
14 a. (3 pts)What three molecular characteristics enable fluorescein to emite fluorescence light?
b.(1 pts) If the following image of a cell labeled with fluorecein shows the cell as being red
in color, what color light does fluorescein absorb?
15. a.(3 pts) Draw the Lewis structure of water and use it to explain why water molecules
absorb IR radiation.
b. (2 pts) Use the concept of formal charge to explain why the hydronium ion (H3O+) has
a + charge.
16. (6 pts) Using the theory of molecular orbitals along with figures to explain why the electrons
in π bonds are easier to excite than the electrons in σ bonds.
17. (5 pts) Using what you know about the energy required to excite electrons in molecules,
rationalize the relative wavelength trend in the functional groups listed below for UV
spectroscopy.
carboxyl group
204nm
CH3COH
carbonyl group
O
186nm
CH3CCH3
alkene
C6H13CH=CH2
177nm
Only do either 18 or 19
18. (5 pts) Devise an experiment to study the diffusion of molecules from a cell’s membrane to
a cell’s nucleus. What technique would you use and how does it work?
19.(5 pts) Devise an experiment to study a conformational change in a molecule (from
conformation A to conformation B). What technique would you use and how does it work?
Conformation B
Conformation A
BONUS (+15 Points) – Partial Credit Will Be Given!!!
How much mass in grams of Magnesium Chloride will you produce from mixing 10.0 mL of
37.2wt% HCl (density 1.19 g/mL) with 10.0 mL of 25.5 wt% Mg(OH)2 (density 1.30 g/mL)?