Quiz5 - threonine chloride . If so, pH at 16 mL should be...

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Chemistry 223 Quantitative Analysis Laboratory Post lab Quiz 10/1/10 Answer Key Questions are based on the following simulated data. 1. How many endpoints could be detected using a visual indicator instead of a pH meter (1)? Explain (1) There is only one sharp endpoint with a pH jump sufficient to give a sharp indicator transition. The pK a of the indicator should be about 6. The endpoint near 65 mL jumps only 2 pH units or so, so the indicator transition would be fuzzy and imprecise. 2. Given that the NaOH used to carry out the titration was 0.1033 M and the weight of unknown titrated was 0.5041 g, what is the equivalent weight of the unknown? (2) Equivalence point appears to be at 32.00 mL. 0.1033 M * 0.03200 L = 0.0033056 moles. 0.5041 g/0.0033056 moles = 152.5 g/mol. That pretty well identifies this as
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Unformatted text preview: threonine chloride . If so, pH at 16 mL should be pK 1 , 2.09 (vs. observed 2.2) and at 1.5 times the equivalence point, it should be pK 2 , 9.1 (vs. observed 9.1). Because the pK 2 's are close together, they are a poor way to distinguish these analytes; in fact the molecular weight/equivalent weight is the best way to do it. 3. If the unknown is one of the following 3 substances, which one is it? (1) Phenylalanine chloride, MW 201.626, pK 1 = 2.2, pK 2 = 9.31 Threonine chloride, MW 152.528, pK 1 = 2.09, pK 2 = 9.1 Tryptophan chloride, MW 240.663, pK 1 = 2.37, pK 2 = 9.33. V(mL) pH 0.00 1.54 10.87 2.00 16.20 2.23 24.73 2.70 31.98 5.30 32.01 6.00 34.35 8.00 48.00 9.10 60.50 10.00 63.90 10.80...
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This note was uploaded on 01/06/2011 for the course CHEM 223 taught by Professor Scheeline during the Fall '08 term at University of Illinois at Urbana–Champaign.

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