Chem 101B: Problem Set 2 Zumdahl, Chapter 8 (Acid-base Problems) Due: Thursday, September 9, 2010 These are problems covering material that will be on the first exam. I recommend that you keep a copy of your work for studying, as these are due one class before the exam, so you will not get your graded problems back before taking the exam. There will be other Ch. 8 problems on solubility equilibria. They will appear on the next problem set. Problems from the textbook: It is crucial that you do these, but you do not have to hand them in for grading. They give you extremely helpful practice for mastering buffers and titration curves. You’ll find it much easier to do the problems that must be handed in if you do these first! Please note: At the end of the lecture on Thursday, August 26, in my rushed introduction of titration curves, some of the details were incorrect. Please read section 8.5 if you start working on the titration curve problems before the lecture on Tuesday, August 31. 6th edition: 16, 19, 21, 37, 41, 46, 53, 57, 59, 64, 68, 75acegi, 79, 115 5th edition: 17, 19, 21, 33, 37, 43, 45, 49, 51, 57, 59, 67acegi, 69, 99 Problems to hand in for grading: 1. a. From your list of weak acids distributed in class, which acid would be optimal for making a buffer with pH = 8.25? Explain your answer. b. If you had 0.35 M solutions of HA and NaA for the acid you chose in Part a, how many milliliters of each would you need to make 100.0 mL of the pH 8.25 buffer? 2. Lactic acid, pKa = 3.85, is an important molecule in biological systems. During exercise, its concentration increases in the blood. a. A biochemist prepares a lactate buffer by mixing 225 mL of 0.85 M lactic acid with 435 mL of 0.68 sodium lactate. What is the pH of this buffer? b. The normal pH of blood is 7.40, which is controlled, in part, by the H2PO4–/HPO42– buffer system. Calculate the concentration ratio ! [HPO42–][H2PO4–] for this buffer at normal blood pH. c. The main buffer in blood plasma is the carbonic acid/bicarbonate system. If the concentration of bicarbonate in blood is 0.0020 M, what must the concentration of CO2(aq) be?PS 2, page 2 3. Find the pH of the following solutions generated from the materials in the bottles shown below: Bottle A 200.0 mL of 0.0750 M HCl Bottle B 200.0 mL of 0.200 M hydrazine (H2N-NH2, pKb = 5.88) Bottle C 100.0 g of solid hydrazine hydrochloride a. Solution #1: 6.80 g of the solid in Bottle C dissolved in water to give a total volume of 500.0 mL b. Solution #2: Mix 75.0 mL of Solution #1 with 75.0 mL of the solution in Bottle B. c. Solution #3: Mix 15.0 mL of the solution in Bottle A with 50.0 mL of Solution #2. 4. Consider the diprotic acid H2A, with pKa1 = 2.75 and pKa2 = 6.25. H2A is colorless, HA– is blue, and A2– is yellow. a. When a 50.0 mL sample of 0.0950 M H2A is titrated with 0.0950 M NaOH, the following curve is obtained. For all of the equivalence and halfway points, label the axes and write in the corresponding pH values and volumes. b. What color is the solution when the pH is 9.25? Explain. c. At what pH will the solution appear most blue? Explain. d. At what pH will the solution appear green?