Question

Fe²⁺ is present at very low concentrations in seawater. The primary source of this iron is Fe(OH)₂. The pH of seawater is between 7.4 and 8.3. For this problem assume that the pH of seawater is 8.0.

a) Write a chemical equation that describes the solubility of Fe(OH)_2.

b) Knowing that ΔGº for this reaction is 90.7 kJ/mol, calculate the K_sp of Fe(OH)₂.

c) What is the solubility of Fe(OH)₂ in pure water?

d) What is the solubility of Fe(OH)₂ in seawater?

I need to know how to do these questions not just the answers, thank you!

Answer 1

a)

Fe(OH)2 (s) + H2O (l) <------------------> Fe+2 (aq) + 2 OH- (aq)

Ksp = [Fe+2][OH-]^2

b)

ΔGº = 90.7 kJ/mol

ΔGº = - R T ln Ksp

90.7 = - 8.314 x 10^-3 x 298 x ln Ksp

Ksp = 1.27 x 10^-16

c)

Fe(OH)2 (s)   <-----------------> Fe+2 + 2 OH-

                                                       S             2S

Ksp = [Fe+2][OH-]^2

1.27 x 10^-16 = S x (2S)^2

S = 3.17 x 10^-6 M

solubility of Fe(OH)₂ in pure water = 3.17 x 10^-6 M

d)

pH = 8.0

pOH = 6.0

[OH-] = 1 x 10^-6 M

Ksp = [Fe+2][OH-]^2

1.27 x 10^-16 = S x (1 x 10^-6)^2

S = 1.27 x 10^-4 M

solubility of Fe(OH)₂ in seawater = 1.27 x 10^-4 M