Iron can be determined gravimetrically by precipitating as Fe(OH)3 and igniting to Fe2O3. The sample to be analyzed is weighed and transferred to a 400-mL beaker where it is dissolved 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 concentrated HNO3. After boiling to remove the oxides of nitrogen, the solution is diluted to 200 mL, brought to boiling, and Fe(OH)3 is precipitated by slowly adding 1:1 NH3 until the odour of NH3 is detected. The solution is boiled for an additional minute, and the precipitate is allowed to settle to the bottom of the beaker. The precipitate is then filtered and washed with several portions of hot 1% w/v NH4NO3 until no Cl– is found in the wash water. Finally, the precipitate is ignited to constant weight at 500–550 °C, and weighed as Fe2O3. (a) If the ignition is not carried out under oxidizing conditions (plenty of O2 present), the final product will contain some Fe3O4. What effect would this have on the reported %w/w Fe? (b) The precipitate is washed with a dilute solution of NH4NO3. Why is NH4NO3 added to the wash water? (c) Why does the
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