HMWK 6 FA 2015(2)

# HMWK 6 FA 2015(2) - C211 FA 2015 HOMEWORK 6 DUE FRIDAY...

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C211 FA 2015 HOMEWORK 6 NAME ___________________________ DUE FRIDAY October 23 BY 5 PM 16 points total. 1. (1.2 pts) A 0.0500 M solution of glycine, an amino acid, is titrated with 0.0500 M NaOH. Two equivalence points are observed as shown in the illustration. They occur at pH = 6.06 and pH = 11.00. What are the two pK a 's for this amino acid? pH mL NaOH added pK a1 = pK a2 = 2. (a) (1.6 points) A triprotic acid has pK's = 3.14, 6.92 and 10.27. Compute the fraction of all four species when the pH = 7.00. The species are H 4 A, H 3 A , H 2 A 2 and A 3 . 0 = 1 = 2 = 3 =

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b. (0.4 pts) If the total analytical concentration, C T , of the acid system in part (2a) is 2.50 x 10 3 M, what is the concentration of each species? [H 3 A] = [H 2 A ] = [HA 2 ] = [A 3 ] = 3. (a) (0.5 pt) The amino acid histidine has pKa’s of 1.7, 6.02 and 9.08. In the grid provided, sketch the plot of alpha functions vs. pH, for this system. Label each curve appropriately as 0 , 1 , 2 , or 3 . 0 1 2 3 4 5 6 7 8 9 10 11 12 13 pH (b) (0.3 pt) Let H 3 A 2+ represent histidine (there are two protonatable N-groups, hence the 2+ charge on the "acid only" form). Fill in the table to show which -value represents which species. The first one is shown as an example. alpha-value 0 1 2 3 Species H 3 A 2+ (c) (0.2 pt) Estimate from your graph the fraction of the species HA when the pH is 6.75. CLEARLY identify this point on your graph, and STATE the value here. ________________ (d) (0.2 pt) At what two pH’s is the species H 2 A + about 70 % of C histidine ?
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