Question

Will iron(III) hydroxide precipitate from a pH = 1.00 solution with [Fe3+] = 1.0 x 10^-5? Ksp Fe(OH)3 = 6.3 x 10^-38

Answer 1

The solubility product constant, Ksp​, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution.

aA(s)⇌cC(aq)+dD(aq)

To solve for the Ksp it is necessary to take the molarities or concentrations of the products (cC and dD) and multiply them. If there are coefficients in front of any of the products, it is necessary to raise the product to that coefficient power(and also multiply the concentration by that coefficient). This is shown below:

Ksp=[C]c[D]d

Solids are not included when calculating equilibrium constant expressions, because their concentrations do not change the expression

When solving this type of exercises, it must be calculated the Kf (formation constant). If Kf is greter than Ksp, there will be precipidated, it Ksp is greater than Kf, it wont happen.

Fe(OH)3 (s) ⇌ [Fe3+]^3 + [HO-]

Kf = [Fe3+]^3x[HO-]

As we have iron concentration, we just need hydroxide concentration. For that, we have the pH

pOH = -log[HO-]

pH + pOH = 14 pOH = 14 - pH pOH = 14 - 1 pOH = 13

[HO-] = e^-pOH [HO-] = e^-13 [HO-] = 2.26x10^-6 mol/L

Kf = (1.0x10^-5)^3 x (2.26x10^-6) Kf = (1.0x10^-15) x (2.26x10^-6) Kf = 2.26x10^-21

2.26x10^-21 is greater than 6.3 x10^-38

As Kf is greater than Ksp, the iron(III) hydroxide will precipitate