Critical Thinking Questions Quickly review questions a through c below What is t

Question

Critical Thinking Questions Quickly review questions a through c below What is the letter on the model titration curve of HPr that corresponds to the point where pH equals the numerical value of the pKa for HPr? a. b. Is the titration curve horizontal or vertical at this point that corresponds to pH = pKa? c. What species of molecules are present at the point noted in question 1b and if possible indicate the percentages? 2. The textbook values for pKas for each ionizable group of cysteine are variable. For the purpose of this exercise, use the values of: 1.96 (Ca-COOH), 8.18 (R group, Ca-SH), and 10.28 (Ca-NH). a. Will the titration curve for cysteine be horizontal or vertical at the points where pH = pk.? b. What species of molecule are present at pH values below, in between, and above the different pKa values? Draw them.

Explanation / Answer

1.

a. At letter B on the curve the numerical value of pH will correspond to its pKa. This is the point where the number of conjugate acid [HPr] and it's conjugate base [Pr-] are equal.

b. When pH= pKa, the curve is horizontal. It is the buffering point, and the pH will not change inspite of adding of NaOH for till some extent as it will combine with H+ released by HPr to form water.

c. HPr, H+ and Pr- are present, all in equal amounts i.e. 50%each.

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