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Midterm

55-237 Study Guide - Midterm Guide: Glutamine, Acid Dissociation Constant, PhenylalanineExam


Department
Biological Sciences
Course Code
BIOL 2070
Professor
Professor Noel
Study Guide
Midterm

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Page 1 of 6
03-59-261 – Organic Chemistry of Biomolecules
Winter 2010
Term Test #1
Student Name:___________________________
Student #: ______________________________
PLEASE ANSWER ALL QUESTIONS
Duration of the test = 80 min.
MARK OBTAINED:
Please Note: Tests written in PENCIL can’t be returned for re-marking.
pKapKapKa
AminoAcid
M
rα‐COOH α‐NH2SideChain
Ala892.349.69
Arg1732.179.0412.48
Asn1322.028.80
Asp1331.889.603.65
Cys1211.9610.288.18
Glu1472.199.674.15
Gln1462.179.13
Gly752.349.60
His1551.829.176.00
Ile1312.369.68
Leu1312.369.60
Lys1462.188.9510.53
Met1492.289.21
Phe1651.839.13
Pro1151.9910.96
Ser1052.219.15
Thr1192.119.62
Trp2042.389.39
Tyr1812.209.1110.07
Val1172.329.62

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Page 2 of 6
Question 1 A mixture of [HA] = 0.05 M and [A-] = 0.01 M of an unknown amino acid had a
pH of 7.5.
i) Identify the amino acid.
[4 marks]
pH = pKa + log([A-]/[HA]) [1 mark]
7.5 = pKa + log(0.01/0.05) [1 mark]
pKa = 7.5 + 0.7 = 8.2 [1 mark]
The amino acid is cysteine [1 mark]
ii) What would the pH of this solution be upon addition of 0.03 M NaOH?
[4 marks]
pH = 8.2 + log([0.01+0.03]/[0.05-0.03]) [2 marks]
pH = 8.2 + 0.3 = 8.5 [2 marks]
Note: if the students calculate the incorrect pKa in (i), they can still get full marks
for (ii) if they do everything else correctly here.
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