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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