Question

What is the molar solubility of fe(oh)3 in pure water? (ksp for fe(oh)3 is 4.0 x 10-38.)

a. 6.5 x 10-7 m
b. 4.5 x 10-10 m
c. 4.0 x 10-17 m
d. 2.0 x 10-10 m
e. 3.4 x 10-13 m?

Answer 1

A) Pure water (assume that the pH is 7.0 and remains constant).
[OH-] = 10⁻⁷
[Fe(3+)] = 4.0x10⁻³⁸/[OH-]³ = 4.0x10⁻¹⁷ (M)
That means the solubility is 4.0x10⁻¹⁷ mol/L.

(B) A solution buffered at pH = 5.0.
[OH-] = 10⁻⁹
[Fe(3+)] = 4.0x10⁻³⁸/[OH-]³ = 4.0x10⁻¹¹ (M)
That means the solubility is 4.0x10⁻¹¹ mol/L.

(C) A solution buffered at pH = 11.0.
[OH-] = 10⁻³
[Fe(3+)] = 4.0x10⁻³⁸/[OH-]³ = 4.0x10⁻²⁹ (M)
That means the solubility is 4.0x10⁻²⁹ mol/L.

Answer 2

The molar solubility of Fe(OH)₃ in pure water is 2.0x10⁻¹⁰ M.

Step-by-step explanation:

First, remember that the molar solubility of a compound is the maximum molar concentration of solute in a specific solvent. In this case, the solute is Fe(OH)₃ and the solvent is water.

- To calculate the molar solubility of a compound, we have to start by writing the dissociation reaction and make sure it is balanced. You can check the attachment for better understanding.

- The molar solubility of each dissociated species is represented by the letter "s" and multiply by its coefficient.

- The molar solubility is calculated from the Ksp of a compound. The Ksp is the Solubility Product Constant, so we have to make the product of molar solubility of each specie:

Kps = [Fe⁺³].[OH⁻]³

It is very important to raise to the power indicated by the coefficient the solubility of each specie when we write it in the Ksp equation.

After a few mathematical steps for clearing the solubility "s" we find that the result is 1.96x10⁻¹⁰ M that it is approximately to 2.0x10⁻¹⁰ M.

Note:

The molar solubility has units of molarity which is represented in chemistry by capital letter M not small letter m. Small letter m represents molality in chemistry. Molarity and molality are differents, be careful.