Analytical Chem istry - DePauw University

524 Analytical Chemistry 2.043. Calculate or sketch titration curves for the following (unbalanced) redoxtitration reactions at 25 o C. Assume the analyte is initially presentat a concentration of 0.0100 M and that a 25.0-mL sample is taken foranalysis. The titrant, which is the underlined species in each reaction,is 0.0100 M.(a) V 2 +Ce 4 +V 3 +Ce3 +( aq) + ( aq) → ( aq) + ( aq)(b) Ti 2 +Fe 3 +Ti 3 +Fe2 +( aq) + ( aq) → ( aq) + ( aq)2+ − 3+ 2+(c) Fe ( aq) + MnO ( aq) → Fe ( aq) + Mn ( aq ) (atpH = 1)444. What is the equivalence point for each titration in problem 43?45. Suggest an appropriate indicator for each titration in problem 43.Some of the problems that follow require one ormore equilibrium constants or standard state potentials.For your convenience, here are hyperlinksto the appendices containing these constantsAppendix 10: Solubility ProductsAppendix 11: Acid Dissociation ConstantsAppendix 12: Metal-Ligand Formation ConstantsAppendix 13: Standard State Reduction Potentials46. The iron content of an ore can be determined by a redox titration usingK 2 Cr 2 O 7 as the titrant. A sample of the ore is dissolved in concentratedHCl using Sn 2+ to speed its dissolution by reducing Fe 3+ to Fe 2+ . Afterthe sample is dissolved, Fe 2+ and any excess Sn 2+ are oxidized to Fe 3+and Sn 4+ using MnO 4 – . The iron is then carefully reduced to Fe 2+ byadding a 2–3 drop excess of Sn 2+ . A solution of HgCl 2 is added and, ifa white precipitate of Hg 2 Cl 2 forms, the analysis is continued by titratingwith K 2 Cr 2 O 7 . The sample is discarded without completing theanalysis if a precipitate of Hg 2 Cl 2 does not form, or if a gray precipitate(due to Hg) forms.(a) Explain why the analysis is not completed if a white precipitate ofHg 2 Cl 2 forms, or if a gray precipitate forms.(b) Is a determinate error introduced if the analyst forgets to add Sn 2+in the step where the iron ore is dissolved?(c) Is a determinate error introduced if the iron is not quantitativelyoxidized back to Fe 3+ by the MnO 4 – ?47. The amount of Cr 3+ in an inorganic salt can be determined by a redoxtitration. A portion of sample containing approximately 0.25 g of Cr 3+is accurately weighed and dissolved in 50 mL of H 2 O. The Cr 3+ isoxidized to Cr 2 O 7 2– by adding 20 mL of 0.1 M AgNO 3 , which servesas a catalyst, and 50 mL of 10%w/v (NH 4 ) 2 S 2 O 8 , which serves as theoxidizing agent. After the reaction is complete the resulting solutionis boiled for 20 minutes to destroy the excess S 2 O 8 2– , cooled to roomtemperature, and diluted to 250 mL in a volumetric flask. A 50-mLportion of the resulting solution is transferred to an Erlenmeyer flask,treated with 50 mL of a standard solution of Fe 2+ , and acidified with200 mL of 1 M H 2 SO 4 , reducing the Cr 2 O 7 2– to Cr 3+ . The excessFe 2+ is then determined by a back titration with a standard solution