Fe 2+ is present at very low concentrations in seawater. The primary source of t

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 Gº for this reaction is 90.7 kJ/mol, calculate the Ksp 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!

Explanation / Answer

a)

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

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

b)

Gº = 90.7 kJ/mol

Gº = - 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

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