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1400-CH08  9/9/99  2:18 PM  Page 270





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              270    Modern Analytical Chemistry
                     Modern Analytical Chemistry
                 combustion analysis. When a 0.5123-g sample was carried  34. Suppose that you have a mixture of CaC 2 O 4 , MgC 2 O 4 , and
                 through the analysis, 1.2119 g of CO 2 and 0.2482 g of H 2 O  some inert impurities, and that you wish to determine the
                 were collected. (b) What are the %w/w C and %w/w H in   %w/w CaC 2 O 4 in the sample. One way to accomplish this
                 this compound? (c) What is the compound’s empirical  analysis is by volatilization gravimetry. The following
                 formula?                                           information is available to you as the result of a
              32. A polymer’s ash content is determined by placing a weighed  thermogravimetric study: CaC 2 O 4 converts to CaCO 3 at a
                 sample in a Pt crucible that has been previously brought to a  temperature of 500 °C; CaCO 3 converts to CaO at a
                 constant weight. The polymer is melted under gentle heating  temperature of 900 °C; and MgC 2 O 4 converts to MgO at a
                 from a Bunsen burner until the volatile vapor ignites. The  temperature of 500 °C. (a) Describe how you would carry out
                 polymer is allowed to burn until only a noncombustible  this analysis, paying particular attention to the measurements
                 residue remains. The residue is then brought to constant  that you need to make. (b) How would you calculate the
                 weight at 800 °C in a muffle furnace. The following data were  %w/w CaC 2 O 4 in the sample?
                 collected during the analysis of two samples of a polymer  35. The concentration of airborne particulates in an industrial
                 resin:                                             workplace was determined by pulling the air through a single-
                                                                    stage air sampler equipped with a glass fiber filter. The air was
                                         g crucible +  g crucible +  sampled for 20 min at a rate of 75 m /h. At the end of the
                                                                                              3
                 Polymer A    g crucible  polymer   ash             sampling period the glass fiber filter was found to have
                                                                    increased in mass by 345.2 mg. What is the concentration of
                 replicate 1  19.1458    21.2287    19.7717
                                                                    particulates in the air sample in milligrams per cubic meter
                 replicate 2  15.9193    17.9522    16.5310
                                                                    and in milligrams per liter?
                 replicate 3  15.6992    17.6660    16.2909
                                                                 36. The fat content of potato chips can be determined indirectly
                                                                    by weighing a sample before and after extracting the fat with
                                         g crucible +  g crucible +  supercritical CO 2 . The following data were obtained for the
                 Polymer B    g crucible  polymer   ash                                         16
                                                                    analysis of one sample of potato chips.
                 replicate 1  19.1457    21.0693    19.7187
                 replicate 2  15.6991    17.8273    16.3327           Sample       Initial Sample   Final Sample
                                                                      Number       Weight (g)       Weight (g)
                 replicate 3  15.9196    17.9037    16.5110
                 (a) Determine the average and standard deviation for the  1         1.1661            0.9253
                 %w/w ash of each polymer resin. (b) Is there any evidence at  2     1.1723            0.9252
                 a= 0.05 for a significant difference between the two polymer  3     1.2525            0.9850
                 resins?                                                4            1.2280            0.9562
              33. In the presence of water vapor the surface of zirconia, ZrO 2 ,  5  1.2837           1.0119
                 chemically adsorbs H 2 O, forming surface hydroxyls, ZrOH
                 (additional water is physically adsorbed as H 2 O). When heated  (a) Determine the average fat content (%w/w) for this sample
                 above 200 °C the surface hydroxyls convert to H 2 O(g), with one  of potato chips. (b) This sample of potato chips is known to
                 molecule of water released for every two surface hydroxyls.  have a fat content of 22.7% w/w. Is there any evidence for a
                 Below 200 °C only physically absorbed water is lost. Nawrocki  determinate error in the data at a= 0.05?
                           15
                 and coworkers used thermogravimetry to determine the  37. Delumyea and McCleary report data for the determination
                 density of surface hydroxyls on a sample of zirconia that was  of %w/w organic material in sediment samples collected at
                 heated to 700 °C and allowed to cool in a desiccator containing   different depths from a cove on the St. Johns River in
                                                                                 17
                 humid N 2 , finding that 0.006 g H 2 O was lost between   Jacksonville, FL. After collecting the sediment core it was
                 200 °C and 900 °C for every gram of dehydroxylated ZrO 2 .  sectioned into a set of 2-cm increments. Each increment
                                                         2
                 Given that the zirconia sample had a surface area of 33 m /g  was placed in 50 mL of deionized water and the slurry
                 and that one molecule of H 2 O forms two surface hydroxyls,  filtered through a piece of preweighed filter paper. The
                 calculate the density of surface hydroxyls in micromoles per  filter paper and sediment increment were then placed in a
                 square meter.                                      preweighed evaporating dish and dried in an oven at
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