D. Harvey - Modern Analytical Chemistry (794078), страница 80
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Explainwhy these experiments produce different results.Experiment 1. When equal volumes of 3.5 M Ba(SCN)2 and3.5 M MgSO4 are mixed, a gelatinous precipitate immediatelyforms.Experiment 2. When equal volumes of 1.5 M Ba(SCN)2 and1.5 M MgSO4 are mixed, a curdy precipitate immediatelyforms. Individual particles of BaSO4 can be seen as pointsunder magnification at 1500×.Experiment 3. When equal volumes of 0.0005 M Ba(SCN)2and 0.0005 M MgSO4 are mixed, the complete precipitationof BaSO4 requires 2–3 h. Individual crystals of BaSO4 obtainlengths of approximately 0.005 mm.7. Aluminum can be determined gravimetrically by precipitatingas Al(OH)3 and isolating as Al2O3.
A sample containingapproximately 0.1 g of Al is dissolved in 200 mL of H2O and5 g of NH4Cl and a few drops of methyl red indicator is added(methyl red is red at pH levels below 4 and yellow at pH levelsabove 6). The solution is heated to boiling, and 1:1 NH3 isadded dropwise till the indicator turns yellow, precipitatingAl(OH)3. The precipitate is held at the solution’s boiling pointfor several minutes, filtered, and washed with a hot solutionof 2%, w/v NH4NO3.
The precipitate is then ignited at1000–1100 °C, forming Al2O3.(a) Cite two ways in which this procedure has been designedto encourage the formation of larger particles of precipitate.(b) The ignition step must be carried out carefully to ensurethat Al(OH)3 is quantitatively converted to Al2O3. What effectwould an incomplete conversion have on the reported %w/wAl? (c) What role do NH4Cl and methyl red indicator play inthis procedure? (d) An alternative procedure involvesisolating and weighing the precipitate as the 8hydroxyquinolate, Al(C9H6ON)3. Why might this be a moreadvantageous form of Al for a gravimetric analysis?8.
Calcium is determined gravimetrically by precipitating it asCaC2O4 H2O, followed by isolating the precipitate as CaCO3.The sample to be analyzed is dissolved in 10 mL of water and15 mL of 6 M HCl. After dissolution, the resulting solution isheated to boiling, and a warm solution of excess ammoniumoxalate is added. The solution is maintained at 80 °C, and 6 MNH3 is added dropwise, with stirring, until the solution isfaintly alkaline.
The resulting precipitate and solution areremoved from the heat and allowed to stand for at least 1 h.After testing the solution for completeness of precipitation, thesample is filtered, washed with 0.1% w/v ammonium oxalate,and dried at 100–120 °C for 1 h.
The precipitate is thentransferred to a muffle furnace where it is converted to CaCO3by drying at 500 ± 25 °C until a constant weight.⋅⋅(a) Why is the precipitate of CaC2O4 H2O converted toCaCO3? (b) In the final step, if the sample is heated at toohigh of a temperature, some CaCO3 may be converted toCaO.
What effect would this have on the reported %w/w Ca?(c) Why is the precipitant, (NH4)2C2O4, added to a hot, acidicsolution rather than to a cold, alkaline solution?1400-CH08 9/9/99 2:18 PM Page 268268Modern Analytical Chemistry9. Iron can be determined gravimetrically by precipitating asFe(OH)3 and igniting to Fe2O3.
The sample to be analyzed isweighed and transferred to a 400-mL beaker where it isdissolved in 50 mL of H2O and 10 mL of 6 M HCl. Any Fe2+that is present is oxidized to Fe3+ with 1–2 mL of concentratedHNO3. After boiling to remove the oxides of nitrogen, thesolution is diluted to 200 mL, brought to boiling, andFe(OH)3 is precipitated by slowly adding 1:1 NH3 until theodor of NH3 is detected. The solution is boiled for anadditional minute, and the precipitate is allowed to settle tothe bottom of the beaker. The precipitate is then filtered andwashed with several portions of hot 1% w/v NH4NO3until no Cl– is found in the wash water. Finally, theprecipitate is ignited to constant weight at 500–550 °C,and weighed as Fe2O3.(a) If the ignition is not carried out under oxidizingconditions (plenty of O2 present), the final product willcontain some Fe3O4.
What effect would this have on thereported %w/w Fe? (b) The precipitate is washed with a dilutesolution of NH4NO3. Why is NH4NO3 added to the washwater? (c) Why does the procedure call for adding NH3 untilthe odor of ammonia is detected? (d) Describe how you mighttest the filtrate for Cl–.10. Sinha and Shome described a gravimetric method formolybdenum in which it is precipitated with nbenzoylphenylhydroxylamine, C13H11NO2, giving aprecipitate of MoO2(C13H10NO2)2.12 The precipitate isweighed after igniting to MoO3. As part of their study theydetermined the optimum conditions for the analysis.Samples containing a known amount of Mo were takenthrough the procedure while varying the temperature, theamount of precipitant added, and the pH of the solution.The solution volume was held constant at 300 mL for allexperiments.
A summary of their results are shown in thefollowing tableg Motaken0.07700.07700.07700.07700.07700.07700.07700.07700.07700.07700.0770TemperaturegmL 10 M(°C)precipitantHClg MoO330303030303030305075800.200.300.350.420.420.420.420.420.420.420.420.90.90.90.90.318.048.075.00.90.90.90.06750.10140.11400.11550.11500.11520.11600.11590.11560.11580.1129Considering these results, discuss the optimum conditionsfor the determination of Mo by this method. Expressyour results for the precipitant as the minimumconcentration in excess, as %w/v, needed to ensure aquantitative precipitation.11. A sample of an impure iron ore is believed to beapproximately 55% w/w Fe.
The amount of Fe in the sampleis to be determined gravimetrically by isolating it as Fe2O3.How many grams of sample should be taken to ensure thatapproximately 1 g of Fe2O3 will be isolated?12. The concentration of arsenic in an insecticide can bedetermined gravimetrically by precipitating it asMgNH4AsO4. The precipitate is ignited and weighed asMg2As2O7.
Determine the %w/w As2O3 in a 1.627-g sample ofinsecticide that yields 106.5 mg of Mg2As2O7.13. After preparing a sample of alum, K2SO4 • Al2(SO4)3 • 24H2O,a student determined its purity gravimetrically. A 1.2931-gsample was dissolved and the aluminum precipitated asAl(OH)3. The precipitate was collected by filtration, washed,and ignited to Al2O3, yielding 0.1357 g. What is the purity ofthe alum preparation?14. To determine the amount of iron in a dietary supplement, arandom sample of 15 tablets weighing a total of 20.505 g wasground into a fine powder. A 3.116-g sample of the powderedtablets was dissolved and treated to precipitate the iron asFe(OH)3.
The precipitate was collected, rinsed, and ignited toa constant weight as Fe2O3, yielding 0.355 g. Report the ironcontent of the dietary supplement as g FeSO4 7H2O pertablet.⋅15. A 1.4639-g sample of limestone was analyzed for Fe, Ca, andMg. The iron was determined as Fe2O3, yielding 0.0357 g.Calcium was isolated as CaSO4, yielding a precipitate of1.4058 g, and Mg was isolated as 0.0672 g of Mg2P2O7. Reportthe amount of Fe, Ca, and Mg in the limestone sample as%w/w Fe2O3, %w/w CaO, and %w/w MgO.16.
The number of ethoxy groups (CH3CH2O–) in an organiccompound can be determined by the following sequence ofreactionsR(OCH2CH3)x + xHI → R(OH)x + xCH3CH2ICH3CH2I + Ag+ + H2O → AgI(s) + CH3CH2OHA 36.92-mg sample of an organic compound with anapproximate molecular weight of 176 was treated in thisfashion, yielding 0.1478 g of AgI. How many ethoxy groupsare there in each molecule?17.
A 516.7-mg sample containing a mixture of K2SO4 and(NH4)2SO4 was dissolved in water and treated with BaCl2,precipitating the SO42– as BaSO4. The resulting precipitate wasisolated by filtration, rinsed free of impurities, and dried to aconstant weight, yielding 863.5 mg of BaSO4. What is the%w/w K2SO4 in the sample?18. The amount of iron and manganese in an alloy can bedetermined by precipitating the metals with 8hydroxyquinoline, C9H7NO. After weighing the mixedprecipitate, the precipitate is dissolved and the amount of8-hydroxyquinoline determined by another method.
In atypical analysis, a 127.3-mg sample of an alloy containingiron, manganese, and other metals was dissolved in acid and1400-CH08 9/9/99 2:18 PM Page 269Chapter 8 Gravimetric Methods of Analysistreated with appropriate masking agents to prevent aninterference from other metals. The iron and manganese wereprecipitated and isolated as Fe(C9H6NO)3 andMn(C9H6NO)2, yielding a total mass of 867.8 mg. Theamount of 8-hydroxyquinolate in the mixed precipitate wasdetermined to be 5.276 mmol.
Calculate the %w/w Fe and%w/w Mn in the alloy.19. A 0.8612-g sample of a mixture consisting of NaBr, NaI, andNaNO3 was analyzed by adding AgNO3 to precipitate the Br –and I–, yielding a 1.0186-g mixture of AgBr and AgI. Theprecipitate was then heated in a stream of Cl2, converting it to0.7125 g of AgCl. Calculate the %w/w NaNO3 in the sample.20. The earliest determinations of elemental atomic weights wereaccomplished gravimetrically. In determining the atomicweight of manganese, a carefully purified sample of MnBr2weighing 7.16539 g was dissolved and the Br– precipitated asAgBr, yielding 12.53112 g. What is the atomic weight for Mnif the atomic weights for Ag and Br are taken to be 107.868and 79.904, respectively?21. While working as a laboratory assistant you prepared 0.4 Msolutions of AgNO3, Pb(NO3)2, BaCl2, KI, and Na2SO4.Unfortunately, you became distracted and forgot to label thesolutions before leaving the laboratory.
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