CHEM 127 TAKE-HOME EXAM 1 : UV-Vis SPECTROSCOPY

PLACE YOUR ANSWERS ON SHORT BOND PAPERS.

Due next week, class time.

1. Make a graph of absorbance versus mole fraction of thiocyanate for the data in the table.

mL Fe3+

solution

mL SCN-

solution

Absorbance

At 455 nm

30.00 0 0.001

27.00 3.00 0.122

24.00 6.00 0.226

21.00 9.00 0.293

18.00 12.00 0.331

15.00 15.00 0.346

12.00 18.00 0.327

9.00 21.00 0.286

6.00 24.00 0.214

3.00 27.00 0.109

0 30.00 0.002

Fe3+

solution: 1.00 mM Fe(NO3)3 + 10.0 mM HNO3;

SCN- solution: 1.00 mM KSCN + 15.0 mM HCl

What is the stoichiometry of the predominant Fe(SCN) n3-n

species?

2. A diene C11H16 was thought to have the structure below. Its UV spectrum showed a max of 263 nm. Can the structure below be correct? If not, draw a structure with the same skeleton that satisfies the spectral data.

3. How do you explain the fact that max for compounds I and II are vastly different whereas those of III and IV are very similar? Answer in a maximum of 3 sentences only.

4. For which of these bases can you use UV-Vis spectroscopy to determine the pKa? Explain your choice (max. 3

sentences only).

5. What limits the resolution of a diode-array spectrophotometer? Why doesnt it normally matter that the resolution is typically 2 nm? Limit your answer to 5 sentences only.

BONUS: Fact or Fiction. Watch the video clip (Einsteins blackboard2.wma) from the TV series, Zero Hour. An instrument was used to reveal Einsteins secret from an erased portion of a blackboard. In a maximum of 10 sentences, comment on whether or not such an instrument exists. Focus your discussion on the basic components

of the instrument and how the instrument was set up.