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CHEM30B_w10_e01_key

# CHEM30B_w10_e01_key - Chemistry 30B Winter 2010 Exam 1...

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Exam 1 Revised 02/08/10 Page 1 Statistics : High score, average, and low score will be posted on the course web site after exam grading is complete. The exam is ready to be picked up when these numbers are posted. Some questions have more than one answer, even though only one answer may be listed here. To see the projected course grade cutoffs, consult the grading scale on the Chemistry 30B course web page. 1. In mass spectrometry, the reason we divide the intensity of the M+1 peak by 1.1% instead of some other percentage is because 1.1% is the natural abundance of 13 C. 2. (a) (4 x 12) + (10 x 1) + 79 + 14 = 151 (b) (4 x 1.107) + (10 x .015) + 0.366% = 4.95, so among the given choices best estimate is 5.0 % (c) 100 % due to bromine 3. (a) Not possible; violates nitrogen rule. (b) Not possible; violates hydrogen rule. (c) Not possible; m/z of M = 138. (d) Possible. Also acceptable, although not as good of an answer: Not possible; calculated relative intensity for M+1 is (8 x 1.107%) + (4 x 0.015%) + (4 x 0.366%) = 10.38%, which is greater than observed M+1 intensity of 9.05%. Recall that ion abundances are empirical numbers and hence have measurement errors. The error in measurement may account for this difference . 4. O O has eight DBE. 5. C–I Increasing the mass of one atom while keeping the other atom the same causes a lower stretching frequency. Example: C–H ~3000 cm -1 , C–C ~ 1450 cm -1 , and C–O ~ 1000 cm -1 . 6. (a) Alcohol: present. Strong, broad peak ~3450 cm -1 . (b) Alkyne: uncertain. Peak ~2150 cm -1 could be alkyne or nitrile . (c) Benzene ring: absent. No peak ~1600 cm -1 . (d) Carboxylic acid: absent. Zone 2 O–H stretch absent; C=O inconsistent . (e) Ester: uncertain. C=O stretch at 1736 cm -1 could be ester or ketone . (f) Ether: uncertain.

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CHEM30B_w10_e01_key - Chemistry 30B Winter 2010 Exam 1...

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