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Test2 Answers - CHM212 Quantitative Chemical Analysis...

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CHM212 Quantitative Chemical Analysis. Test #2, Dec. 1, 2011. All answers must be legible, neatly organized and clearly explained. You must include units for measured values, decimal places must be preceded by a “0” (ie. “0.2” and not “.2”). Explanations must include word- or symbol-labelled equations to define the equations that you are using, before you substitute in numbers. Your answer must be comprehensible based on a quick, single sequential read-through. Failure to adhere to these requirements may result in a mark of 0 for that answer. Answers may be marked based only on the final answer, although even in that case the failure to present a detailed solution will result in a mark of 0. Unless otherwise stated, use the usual approximate substitution of concentration for activity. You may use the Henderson-Hasselbalch equation without derivation. TOTAL: 260 marks 1. [20 marks] Define isoelectric point . Contrast with isoionic point . State one application of the isoelectric point and why/how it works. Isoelectric point=isoelectric pH, the pH at which the average charge of apolyprotic species is 0. For the amino acid leucine, represented by the neutral, zwitterionic HL and having equilibria At the isoelectric point, and . In contrast, the isoionic point (or isoionic pH) is the pH obtained when the pure, neutral polyprotic acid HL is dissolved in water. The only ions in solution are H 2 A + , A - , H + and OH - and . See isoelectric focusing or the textbook and classroom discussion of the surface charge of glass. 2. [10 marks] Define zwitterion. Why is it important? A zwitterions is a species that has two moieties within the molecule bearing opposite formal charges. For an amino acid such as alanine, the zwitterions is the net neutral species. The sites within an amino acid have different pK a ’s allowing their protonation state to change as a function of pH, to make the amino acid overall positive, neutral or negative. The zwitterionic nature of amino acids allows them to
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