when iron hydroxide dissolves, it dissociates as follows
Fe(OH)₂ --> Fe²⁺ + 2OH⁻
molar solubility is the number of moles of solute that can be dissolved is a 1 L solution
if molar solubility of Fe(OH)₂ is x then molar solubility of Fe²⁺ is x and OH⁻ is 2x
the solubility product constant - ksp formula is ;
ksp = [Fe²⁺][OH⁻]²
ksp = (x)(2x)²
ksp = x * 4x²
ksp = 4x³
ksp = 4.87 x 10⁻¹⁷
4x³ = 4.87 x 10⁻¹⁷
x = 2.30 x 10⁻⁶ M
molar solubility of Fe(OH)₂ is 2.30 x 10⁻⁶ M