Question

A solution contains 1.16×10⁻² m iron(iii) nitrate and 1.45×10⁻² m calcium acetate. solid potassium hydroxide is added slowly to this mixture.what is the concentration of hydroxide ion when this precipitation first begins?

Answer 1

The concentration of hydroxide ions when precipitation first begins can be calculated by taking the cube root of the solubility product constant (Ksp) divided by the iron(III) ion concentration, which is the same as the initial concentration of iron(iii) nitrate.

Step-by-step explanation:

To determine the concentration of hydroxide ion when the precipitation first begins, we first need to consider the solubility product constant (Ksp) for Fe(OH)3, which represents the equilibrium between solid iron(III) hydroxide and its ions in solution:

Fe(OH)3 (s) ⇌ Fe3+ (aq) + 3OH− (aq)
The Ksp for Fe(OH)3 is 4.87 × 10−17. From the initial concentration of iron(III) nitrate, we can assume that the concentration of Fe3+ is the same, as the nitrate ion does not participate in the precipitation reaction. The solubility product expression for the precipitation of Fe(OH)3 is given by:

Ksp = [Fe3+][OH−]3

The concentration of hydroxide ions can be found by taking the cube root of the Ksp divided by the iron(III) ion concentration:

[OH−] = ∓(Ksp / [Fe3+])
By plugging in the values, we get [OH−] = ∓(4.87 × 10−17 / 1.16 × 10−2). Calculation of this value will yield the concentration of hydroxide ions when precipitation first starts as Fe(OH)3 begins to form.