Question

What is the pH of a saturated solution of Fe(OH)2 at 25oC? For Fe(OH)2, Ksp=7.9x10-16. Give three digits in your answer (e.g. 8.47, 12.6, 3.75, etc.)

Answer 1

Let S(mol/L) be the solubility of Fe(OH)₂

Fe(OH)₂ Fe²⁺ + 2OH^-

1 mole of Fe(OH)₂ produces 1 mole of Fe²⁺ & 2 moles of OH^-

Then [Fe²⁺ ] = S

        [OH^-] = 2S

Then solubility product constant , K_sp = [Fe²⁺ ][OH^-]²

                                     7.9x10^-16 = S x (2S)²

                                              4S³ = 7.9x10^-16

                                               S = 5.82x10^-6 mol/L

Therefore [OH^-] = 2S = 2x 5.82x10^-6 mol/L

                              = 1.16x10^-5 mol/L

We know that pOH = - log[OH-]

                            = - log(1.16x10^-5 )

                           = 4.93

So pH = 14 - pOH

         = 14 - 4.93

         = 9.07

Therefore the pH of the solution is 9.07