A laboratory technician needs to know the solution pH at which Fe(OH)3(s) will begin to precipitate from a 0.0020 M Fe(NO3)3 solution at 25 °C. The dissolution equilibrium of Fe(OH)3(s) is represented by Fe(OH)3(s) ⇌ Fe3+(aq) + 3 OH−(aq), and Ksp=4.0×10−38. Pure water is used to adjust the pH (volumes are additive).
(a) Write the expression for Ksp corresponding to the dissolution equilibrium of Fe(OH)3(s).
(b) Calculate the minimum hydroxide-ion concentration, in mol L−1, required for Fe(OH)3 to begin to precipitate from the 0.0020 M Fe3+ solution.
(c) Determine the minimum pH at which precipitation is initiated.
(d) If enough 0.10 M NaOH is added to raise the solution pH exactly one unit above the value calculated in part (c), predict qualitatively (no calculation required) whether any additional precipitate will form beyond the initial appearance, and explain your reasoning.