Calculate the solubility of Fe(OH)3 in each of the following:

Question

Calculate the solubility of Fe(OH)3 in each of the following:

k_sp = 4 x 10^-38

a. Water

b. A solution buffered at pH = 5.0

c. A solution buffered at pH = 11.0

when I calculate this I get

(a) equals 1.96 x 10^-11

(b) equals 1.48 x 10^-12

and I didn’t even try for (c) because these answers weren’t correct.

the correct answers are:

a. 4 x 10^-17

b. 4 x 10^-11

c. 4 x 10^-29

please show your steps so I can follow along. Thank you!

Answer 1

a) in water:

We will set [OH^-] = 1x10^-7 and this is because the value of Ksp is so very small that one must take into account the auto ionization of H₂O. answer.

Fe(OH)₃ ==> Fe³⁺ + 3OH^-

Ksp = 4x10^-38 = [Fe³⁺][OH^-]³ = (x)(1x10^-7)³ = 1x10^-21x

x = 4x10^-17 M

b) in pH 5

pH = 5 so pOH = 9 and [OH-] = 1x10^-9

4x10^-38 = [Fe³⁺][1x10^-9]³ = 1x10^-27[Fe³⁺]

[Fe³⁺] = 4x10^-11 M

c) in pH 11

pH = 11 so pOH = 3 and [OH^-] = 1x10^-3

4x10^-38 = [Fe³⁺][1x10^-3]³ = 1x10^-9 [Fe³⁺]

[Fe³⁺] = 4x10^-29 M