For this problem, we’re being asked to calculate the concentration of Fe2+ needed in order to precipitate Fe(OH)2 from a buffer solution with pH 9.99.
Since the compound is an ionic compound, it forms ions when dissociating in water. The dissociation of Fe(OH)2 in water is:
The hydroxide ion, OH–, has a charge of –1. Iron (Fe) then has a charge of +2:
Fe(OH)2 → Fe2+(aq) + 2 OH–(aq)
Above what Fe2+ concentration will Fe(OH)2 precipitate from a buffer solution that has a pH of 9.99? The Ksp of Fe(OH)2 is 4.87.