D. Harvey - Modern Analytical Chemistry (794078), страница 81
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Realizing your error,you labeled the solutions A–E and performed all possiblebinary mixings of the five solutions. The following resultswere obtained where NR means no reaction was observed, Wmeans a white precipitate formed, and Y means a yellowprecipitate formed.AABBCDENRYNRWYWWNRNRCDWIdentify solutions A–E.ZnCl2 K2CO3 MgSO4Ba(C2H3O2)223. Two methods have been proposed for the analysis of sulfur inimpure samples of pyrite, FeS2. Sulfur can be determined in adirect analysis by oxidizing it to SO42– and precipitating asBaSO4. An indirect analysis is also possible if the iron isprecipitated as Fe(OH)3 and isolated as Fe2O3. Which of thesemethods will provide a more sensitive determination forsulfur? What other factors should be considered in decidingbetween these methods?24.
A sample of impure pyrite known to be approximately90–95% w/w FeS2 is to be analyzed by oxidizing the sulfur toSO42– and precipitating as BaSO4. How many grams of thesample must be taken if a minimum of 1 g of BaSO4 isdesired?25. A series of samples consisting of any possible combination ofKCl, NaCl, and NH4Cl is to be analyzed by adding AgNO3 toprecipitate AgCl.
What is the minimum volume of 5% w/vAgNO3 necessary to completely precipitate the chloride in any0.5-g sample?26. When precipitation does not result in a “stoichiometric”precipitate, a gravimetric analysis is still feasible if thestoichiometric ratio between analyte and precipitate can beestablished experimentally. Consider, for example, theprecipitation gravimetric analysis of Pb as PbCrO4. (a) Foreach gram of Pb in a sample, how many grams of PbCrO4 areexpected to form? (b) In a study of this procedure, Grotefound that 1.568 g of PbCrO4 formed per gram of Pb.14 Whatis the apparent stoichiometry between Pb and PbCrO4?(c) Does failing to account for the actual stoichiometry lead toa positive or negative determinate error for the analysis?27.
Determine the uncertainty for the gravimetric analysisdescribed in Example 8.1. (a) How does your result comparewith the expected accuracy of 0.1–0.2% for precipitationgravimetry? (b) What sources of error might account for anydiscrepancy between the most probable measurement errorand the expected accuracy?22. A solid sample is known to consist of approximately equalamounts of two or more of the following soluble salts.AgNO3269NH4NO3A sample of the solid, sufficient to give at least 0.04 mol of anysingle salt, was added to 100 mL of water, yielding a whiteprecipitate and a clear solution. The precipitate was collectedand rinsed with water.
When a portion of the precipitate wasplaced in dilute HNO3, it completely dissolved, leaving acolorless solution. A second portion of the precipitate wasplaced in dilute HCl, yielding a precipitate and a clearsolution. Finally, the filtrate from the original precipitate wastreated with excess NH3, yielding a white precipitate. Indicatewhich of the soluble salts must be present in the sample,which must be absent and those for which there is insufficientinformation to make this determination.1328. A 38.63-mg sample of potassium ozonide, KO3, was heated to70 °C for 1 h, undergoing a weight loss of 7.10 mg.
Write abalanced chemical reaction describing this decompositionreaction. A 29.6-mg sample of impure KO3 experiences a4.86-mg weight loss when treated under similar condition.What is the %w/w KO3 in the sample?29. The water content of an 875.4-mg sample of cheese wasdetermined with a moisture analyzer. What is the %w/w H2Oin the cheese if the final mass was found to be 545.8 mg?30. Method 8.2 describes a procedure for determining Si in oresand alloys. In this analysis a weight loss of 0.21 g correspondsto 0.1 g of Si. Show that this relationship is correct.31.
The iron content of an organometallic compound wasdetermined by treating a 0.4873-g sample with HNO3 andheating to volatilize the organic material. After ignition, theresidue of Fe2O3 was found to weigh 0.2091 g. (a) What is the%w/w Fe in this compound? The carbon and hydrogen in asecond sample of the compound were determined by a1400-CH08 9/9/99 2:18 PM Page 270270Modern Analytical Chemistrycombustion analysis. When a 0.5123-g sample was carriedthrough the analysis, 1.2119 g of CO2 and 0.2482 g of H2Owere collected.
(b) What are the %w/w C and %w/w H inthis compound? (c) What is the compound’s empiricalformula?32. A polymer’s ash content is determined by placing a weighedsample in a Pt crucible that has been previously brought to aconstant weight. The polymer is melted under gentle heatingfrom a Bunsen burner until the volatile vapor ignites. Thepolymer is allowed to burn until only a noncombustibleresidue remains. The residue is then brought to constantweight at 800 °C in a muffle furnace. The following data werecollected during the analysis of two samples of a polymerresin:Polymer Ag crucibleg crucible +polymerg crucible +ashreplicate 1replicate 2replicate 319.145815.919315.699221.228717.952217.666019.771716.531016.2909Polymer Bg crucibleg crucible +polymerg crucible +ashreplicate 1replicate 2replicate 319.145715.699115.919621.069317.827317.903719.718716.332716.5110(a) Determine the average and standard deviation for the%w/w ash of each polymer resin.
(b) Is there any evidence atα = 0.05 for a significant difference between the two polymerresins?33. In the presence of water vapor the surface of zirconia, ZrO2,chemically adsorbs H2O, forming surface hydroxyls, ZrOH(additional water is physically adsorbed as H2O). When heatedabove 200 °C the surface hydroxyls convert to H2O(g), with onemolecule of water released for every two surface hydroxyls.Below 200 °C only physically absorbed water is lost. Nawrockiand coworkers15 used thermogravimetry to determine thedensity of surface hydroxyls on a sample of zirconia that washeated to 700 °C and allowed to cool in a desiccator containinghumid N2, finding that 0.006 g H2O was lost between200 °C and 900 °C for every gram of dehydroxylated ZrO2.Given that the zirconia sample had a surface area of 33 m2/gand that one molecule of H2O forms two surface hydroxyls,calculate the density of surface hydroxyls in micromoles persquare meter.34.
Suppose that you have a mixture of CaC2O4, MgC2O4, andsome inert impurities, and that you wish to determine the%w/w CaC2O4 in the sample. One way to accomplish thisanalysis is by volatilization gravimetry. The followinginformation is available to you as the result of athermogravimetric study: CaC2O4 converts to CaCO3 at atemperature of 500 °C; CaCO3 converts to CaO at atemperature of 900 °C; and MgC2O4 converts to MgO at atemperature of 500 °C. (a) Describe how you would carry outthis analysis, paying particular attention to the measurementsthat you need to make.
(b) How would you calculate the%w/w CaC2O4 in the sample?35. The concentration of airborne particulates in an industrialworkplace was determined by pulling the air through a singlestage air sampler equipped with a glass fiber filter. The air wassampled for 20 min at a rate of 75 m3/h. At the end of thesampling period the glass fiber filter was found to haveincreased in mass by 345.2 mg.
What is the concentration ofparticulates in the air sample in milligrams per cubic meterand in milligrams per liter?36. The fat content of potato chips can be determined indirectlyby weighing a sample before and after extracting the fat withsupercritical CO2. The following data were obtained for theanalysis of one sample of potato chips.16SampleNumber12345Initial SampleWeight (g)1.16611.17231.25251.22801.2837Final SampleWeight (g)0.92530.92520.98500.95621.0119(a) Determine the average fat content (%w/w) for this sampleof potato chips.
(b) This sample of potato chips is known tohave a fat content of 22.7% w/w. Is there any evidence for adeterminate error in the data at α = 0.05?37. Delumyea and McCleary report data for the determinationof %w/w organic material in sediment samples collected atdifferent depths from a cove on the St.
Johns River inJacksonville, FL.17 After collecting the sediment core it wassectioned into a set of 2-cm increments. Each incrementwas placed in 50 mL of deionized water and the slurryfiltered through a piece of preweighed filter paper. Thefilter paper and sediment increment were then placed in apreweighed evaporating dish and dried in an oven at1400-CH08 9/9/99 2:18 PM Page 271Chapter 8 Gravimetric Methods of Analysis110 °C.
After weighing, the filter paper and sediment weretransferred to a muffle furnace where the filter paper andany organic material were removed by ashing. Finally, theinorganic residue remaining after ashing was weighed.Using the following data, determine the %w/w organicmaterial as a function of the average depth for eachincrement.Depth(cm)Weight ofPaperFilter(g)Weight ofEvaporatingDish(g)WeightAfterDrying(g)WeightAfterAshing(g)0–22–44–66–88–1010–1212–1414–1616–181.5901.7451.6191.6111.6581.6281.6331.6301.63643.2140.6241.2342.1043.6243.2443.0843.9643.3652.1048.8352.8650.5951.8849.4547.9258.3154.3749.4946.0047.8447.1347.5345.3144.2055.5352.7538. Yao and associates recently described a method for thequantitative analysis of thiourea based on its reactionwith I2.18CS(NH2)2 + 4I2 + 6H2O → (NH4)2SO4 + 8 HI + CO2The procedure calls for placing a 100-µL aqueous samplecontaining the thiourea in a 60-mL separatory funnel alongwith 10 mL of a pH 7 buffer and 10 mL of a 12 µM solution of271I2 in CCl4.
The contents of the separatory funnel are shaken,and the organic and aqueous layers are allowed to separate.The organic layer, containing the excess I2, is transferred tothe surface of a piezoelectric crystal on which a thin layer ofAu has been deposited. After allowing the I2 to adsorb to theAu, the CCl4 is removed and the crystal’s frequency shift ismeasured.
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