Analytical Chem istry - DePauw University

398 Analytical Chemistry 2.0(a) If ignition is not carried out under oxidizing conditions (plentyof O 2 present), the final product may contain Fe 3 O 4 . What effectwould this have on the reported %w/w Fe?(b) The precipitate is washed with a dilute solution of NH 4 NO 3 . Whyis NH 4 NO 3 added to the wash water?(c) Why does the procedure call for adding NH 3 until the odor of ammoniais detected?(d) Describe how you might test the filtrate for Cl – .11. Sinha and Shome described a gravimetric method for molybdenum inwhich it is precipitated as MoO 2 (C 13 H 10 NO 2 ) 2 using n-benzoylphenylhydroxylamine,C 13 H 11 NO 2 , as a precipitant. 12 The precipitate isweighed after igniting to MoO 3 . As part of their study, the authors determinedthe optimum conditions for the analysis. Samples containing0.0770 g of Mo were taken through the procedure while varying thetemperature, the amount of precipitant added, and the pH of the solution.The solution volume was held constant at 300 mL for all experiments.A summary of their results are shown in the following table.Temperature( o C)Mass (g)of precipitantVolume (mL)of 10 M HClMass (g)of MoO 330 0.20 0.9 0.067530 0.30 0.9 0.101430 0.35 0.9 0.114030 0.42 0.9 0.115530 0.42 0.3 0.115030 0.42 18.0 0.115230 0.42 48.0 0.116030 0.42 75.0 0.115950 0.42 0.9 0.115675 0.42 0.9 0.115880 0.42 0.9 0.1129Considering these results, discuss the optimum conditions for determiningMo by this method. Express your results for the precipitant asthe minimum %w/v in excess, needed to ensure a quantitative precipitation.12. A sample of an impure iron ore is approximately 55% w/w Fe. Theamount of Fe in the sample is to be determined gravimetrically byisolating it as Fe 2 O 3 . What mass of sample do you need to ensure thatyou isolate at least 1 g of Fe 2 O 3 ?12 Sinha, S. K.; Shome, S. C. Anal. Chim. Acta 1960, 24, 33–36.