1Chemistry 222 Name__________________________________________ Fall 2007 Exam 3: Chapters 9-12 80 Points Complete five (5) of the following problems. Each problem is worth 16 points. CLEARLY mark the problems you do not want graded. You must show your work to receive credit for problems requiring math. Report your answers with the appropriate number of significant figures. 1. Consider the titration curve below. The curve is the result of the titration of 20.00 mL of a 0.100 M solution of one of the acids below with 0.100 M NaOH. Identify the acid. Clearly justify your decision. Acid pKa’s Acetic 4.75 Phosphoric 2.17, 7.20, 12.15Succinic 4.21, 5.64 HCl strong Maleic 1.91, 6.33 Pyruvic 2.55 0.002.004.006.008.0010.0012.0014.0001020304050607080Volume NaOH Delivered (mL)pHFirst, let’s look at the experimental conditions. Since the concentration of the acid and base are the same, we’d expect to see an equivalence point at 20 mL if the acid is monoprotic, 20 and 40 mL if the acid is diprotic (and the Ka’s are different enough), and at 20, 40, and 60 mL if the acid is triprotic (and the Ka’s are different enough). We see a single clear equivalence point at 40 mL, therefore the acid cannot be monoprotic, ruling out acetic, pyruvic, and HCl. The remaining options are polyprotic acids. What would we expect from each? Phospohoric Acid: Since Ka1and Ka2differ by ~105, we’d expect to see a clear distinction between the two equivalence points (@ 20 and 40 mL). Since we don’t observe this, the acid isn’t phospohoric. Maleic Acid: The same argument holds in this case as well. Because the Ka’s are quite different, we’d expect to see both equivalence points, since we do not, the acid can’t be maleic. Succinic Acid: The Ka’s are quite similar, therefore we would not expect to see two distinct equivalence points. Instead, we’d expect to see them merge at ~40 mL. This is what we observe in the titration curve. Therefore, the acid must be Succinic. You could do additional calculations to verify this, but it is unnecessary.
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