The iron content of an ore can be determined by a redox titration using K2Cr2O7 as the titrant. A sample of the ore is dissolved in concentrated HCl using Sn2+ to speed its dissolution by reducing Fe3+ to Fe2+. After the sample is dissolved, Fe2+ and any excess Sn2+ are oxidized to Fe3+ and Sn4+ using MnO4-. The iron is then carefully reduced to Fe2+ by adding a 2-3 drop excess of Sn2+. A solution of HgCl2 is added and, if a white precipitate of Hg2Cl2 forms, the analysis is continued by titrating with K2Cr2O7. The sample is discarded without completing the analysis if a precipitate of Hg2Cl2 does not form, or if a gray precipitate (due to Hg) forms. (a) Explain why the analysis is not completed if a white precipitate of Hg2Cl2 forms, or if a gray precipitate forms. (b) Is a determinate error introduced if the analyst forgets to add Sn2+ in the step where the iron ore is dissolved? (c) Is a determinate error introduced if the iron is not quantitatively oxidized back to Fe3+ by the MnO4-?