# 1 - MCB102 Summer 2009 Problem Set 1 Starred questions were...

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MCB102, Summer 2009 - Problem Set 1 Starred questions were NOT exam questions. Questions labeled ‘2008' are from last year’s midterm. Tuesday lecture questions: *1. Draw the titration curve for a weak acid, HA, whose pKa is 3.2. Label the axes. Indicate with an arrow where on the curve the ratio of conjugate base (A-) to protonated acid (HA) is 3:1. What is the pH at this point? 2. Here is the dissociation reaction for lactic acid. The pKa of lactic acid is 3.86. a) When the concentration of lactic acid goes from 1M to 2M, what is the change in the pKa? b) What is the pH of a solution of lactic acid that has been titrated to the point where there is 10 times as much of the conjugate base present as of the protonated acid? c) At that pH, what is the average net charge on lactic acid? 3. (2008) Your lab partner offers to make up the three buffers you need. These three buffers are: Boric Acid (pKa 9.24) Sodium acetate (pKa 4.76 Imidazole (pKa 6.95) You find the three bottles on the lab bench, but they are labeled only Buffer 1, pH 7.0 Buffer 2, pH 7.0 Buffer 3, pH 7.0 Your partner has taken the lab notebook, and her cell phone is on the bench – you can’t call, and you can’t look up the mystery code for buffers 1, 2, and 3. You need to use the Imidazole buffer right away. In order to identify it, you do the following: Put 20ml of Buffer 1 into each of two beakers. To one beaker, add one drop of 5M HCl To the other beaker, add one drop 5M NaOH Read the pH of each beaker. Repeat this procedure for each of the other two buffers. Here are your results: pH in beaker with added HCl pH in beaker with added NaOH Buffer 1: 2.0 8.4 Buffer 2: 6.5 7.1 Buffer 3: 5.2 11.9 a. You now know which Buffer bottle contains Imidazole. Which is it, and why? Precisely describe how the data led you to this conclusion. b. Which compound is present in each of the other two beakers? Explain your reasoning, based on the data above.

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Wednesday lecture questions: *4. Draw each of these structures and label the C and N termini: a) Asp-Gly-Ala b) Trp-Ala-Leu c) Asp-Met-Glu d) Asp-Trp-Tyr e) Leu-Ala-Trp Which is the most negatively charged at pH 7.0? How many negatively charged groups does it contain? Which contain(s) the largest number of aromatic side chains? Which has the greatest absorbance at 280nm? Which two (if any) represent the same compound? Which two (if any) will have identical products when completely hydrolyzed? 5. Consider the pentapeptide shown below. It has two pKas, one of 8.30 and the other of 3.50. a. On the diagram, show the location and identities of the two groups with acid-base behavior that represent each of these pKas (show which is which). Gly-Gln-Asn-Gly-Phe b. The carboxyl terminal amino acid, if it were a free amino acid, would have a carboxyl group pka closer to 2. Explain why the pKa of the carboxyl group is higher when the amino acid is part of this pentapeptide. c. At what pH or pHs would the buffering capacity of this pentapeptide be maximized?
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## This note was uploaded on 09/28/2009 for the course MCB 102 taught by Professor Staff during the Summer '08 term at Berkeley.

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1 - MCB102 Summer 2009 Problem Set 1 Starred questions were...

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