Iron (55.845 g/mol) can be determined gravimetrically by precipitation as Fe(OH)3 (106.867 g/mol) and subsequent inflammation of the precipitate to convert it to Fe2O3 (159.69 g/mol). A 0.7873 g sample to be analyzed was weighed and transferred to a 400 mL beaker, where it was dissolved in 50 mL of H2O and 10 mL of 6M HCl. Then 2 mL of concentrated HNO3 was added to the solution. After boiling to remove excess HNO3 and nitrogen oxides, the solution was diluted to 200 mL, brought to boiling, and Fe(OH)3 was precipitated by the slow addition of 1:1 NH4OH solution until an odor of NH3 was detected (indicating the presence of excess NH3 in the medium). The solution was left in a water bath at 80 ºC for 30 min at rest. The precipitate was then filtered and washed with several portions of hot 1% m/v NH4NO3 until no Cl– was detected in the wash water. Finally, the precipitate was ignited at 500-550 °C, then left in a desiccator to cool and weighed as Fe2O3, resulting in a mass of 0.4512 g.
(a) What is the importance of the step in which the precipitate is left to rest in a water bath for 30 min?
(b) Why is the precipitate washed with a dilute NH4NO3 solution? What could happen if washing was done with hot water?
(c) What qualitative test can be performed to check for the presence of Cl– in the filtrate? Present the
(d) What is the percentage of iron in the analyzed sample?
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