Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods401oxidizing it to SO 4 2– and precipitating it as BaSO 4 . An indirect analysisis possible if the iron is precipitated as Fe(OH) 3 and isolated as Fe 2 O 3 .Which of these methods will provide a more sensitive determinationfor sulfur? What other factors should you consider in choosing betweenthese methods?25. A sample of impure pyrite known to be approximately 90–95% w/wFeS 2 is to be analyzed by oxidizing the sulfur to SO 4 2– and precipitatingit as BaSO 4 . How many grams of the sample must you take to form atleast 1 g of BaSO 4 ?26. A series of samples containing any possible combination of KCl, NaCl,and NH 4 Cl is to be analyzed by adding AgNO 3 and precipitatingAgCl. What is the minimum volume of 5% w/v AgNO 3 necessary tocompletely precipitate the chloride in any 0.5-g sample?27. If a precipitate of known stoichiometry does not form, a gravimetricanalysis is still feasible if we can establish experimentally the mole ratiobetween the analyte and the precipitate. Consider, for example, theprecipitation gravimetric analysis of Pb as PbCrO 4 . 14(a) For each gram of Pb, how many grams of PbCrO 4 should form?(b) In a study of this procedure, Grote found that 1.568 g of PbCrO 4formed for each gram of Pb. What is the apparent stoichiometrybetween Pb and PbCrO 4 ?(c) Does failing to account for the actual stoichiometry lead to a positivedeterminate error or a negative determinate error?28. Determine the uncertainty for the gravimetric analysis described inExample 8.1. The expected accuracy for a gravimetric method is 0.1–0.2%. What additional sources of error might account for the differencebetween your estimated uncertainty and the expected accuracy?29. A 38.63-mg sample of potassium ozonide, KO 3 , was heated to 70 o Cfor 1 h, undergoing a weight loss of 7.10 mg. A 29.6-mg sample ofimpure KO 3 experiences a 4.86-mg weight loss when treated undersimilar condition. What is the %w/w KO 3 in the sample?30. The water content of an 875.4-mg sample of cheese was determinedwith a moisture analyzer. What is the %w/w H 2 O in the cheese if thefinal mass was found to be 545.8 mg?31. Representative Method 8.2 describes a procedure for determining Si inores and alloys. In this analysis a weight loss of 0.21 g corresponds to0.1 g of Si. Show that this relationship is correct.14 Grote, F. Z. Anal. Chem. 1941, 122, 395–398.