Unformatted text preview: Answer Key Chemistry 223 Quantitative Analysis Laboratory Post‐lab Quiz 10/10/10 1. In the fall of 2009, your instructor was asked to review a paper in which the authors claimed to have a method to measure pH spectrophotometrically. The paper had a plot that looked something like the one shown below. The authors fit the line shown on the plot to the data points also shown. "Correlation coefficient is 0.97, and pH can be measured between 3.25 and 5.75 with a linear fit as shown." Would you accept this paper? Explain your answer, based on what you did in lab this week and throughout the semester. (2) The absorbance follows a sigmoidal curve, as it follows the pH transition of an equivalence point of the indicator. IT IS NOT A LINEAR FUNCTION! The authors of this paper should have used the functions ~ A = mα1 + b with α1 = (Ka/aH+)/(1+ Ka/aH+) or α1 = (Ka)/( aH++ Ka). Because the authors didn't understand the material at Chem 222/223 level, the paper should be rejected. 2. If instead of plotting the ratio Apeak/Aisosbestic vs. 0 you plotted Apeak vs 0, what problems would you have? Name 2. (2) a) You'd fail to normalize for changes in indicator concentration. b) b) You'd need to know the molar absorptivities of each indicator form rather than ratios of those absorptivities. 3. According to the Henderson‐Hasselbalch equation, pH should be independent of dilution for a buffer. In reality there is a small change in pH with dilution. Why? (there is a 2 word answer!) (1) "activity correction" or "activity coefficient" ...
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 Fall '08
 SCHEELINE
 pH, 0, Quantitative Analysis Laboratory, pH spectrophotometrically.

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