What is the molar solubility of Fe(OH)2 at 25°C if the Ksp of the compound is 7.9 × 10⁻¹⁶?

• A. 5.6 × 10⁻⁷ M
• B. 2.8 × 10⁻⁸ M
• C. 3.9 × 10⁻⁵ M
• D. 1.3 × 10⁻⁶ M

Answer: (D)

Verified step by step guidance

1. Start by writing the balanced dissolution equation for Fe(OH)₂: Fe(OH)₂(s) ⇌ Fe²⁺(aq) + 2OH⁻(aq). This equation shows that one mole of Fe(OH)₂ dissolves to produce one mole of Fe²⁺ ions and two moles of OH⁻ ions.
2. Define the molar solubility of Fe(OH)₂ as 's'. This means that the concentration of Fe²⁺ ions in solution will be 's' and the concentration of OH⁻ ions will be '2s' due to the stoichiometry of the dissolution reaction.
3. Write the expression for the solubility product constant (Ksp) for Fe(OH)₂: Ksp = [Fe²⁺][OH⁻]². Substitute the concentrations from the previous step into this expression: Ksp = (s)(2s)².
4. Simplify the expression: Ksp = 4s³. This equation relates the Ksp value to the molar solubility 's'.
5. Solve for 's' by substituting the given Ksp value (7.9 × 10⁻¹⁶) into the equation: 7.9 × 10⁻¹⁶ = 4s³. Rearrange the equation to find 's', which represents the molar solubility of Fe(OH)₂.