Distinctive Properties with the Same Ferrous Ion: FeSO<sub>4</sub>&#8729;7H<sub>2</sub>O and (NH<sub>4</sub>)<sub>2</sub>Fe(SO<sub>4</sub>)<sub>2</sub>&#8729;6H<sub>2</sub>O: An Improvement of the Experiment on Preparation of Ferrous Ammonium Sulfate Hexahydrate