Question

Will iron(III) hydroxide precipitate from a pH = 1.00 solution with [Fe3+] = 1.0 x 10-5? Ksp Fe(OH)3 = 6.3 x 10-38

Answer 1

Will iron(III) hydroxide precipitate from a pH = 1.00 solution with [Fe3+] = 1.0 x 10-5? Ksp Fe(OH)3 = 6.3 x 10-38

Dissociation of Fe(OH)₃ is given as,

Fe(OH)₃ ------------> Fe³⁺ (aq.) + 3OH^- (aq.)

Ksp = [Fe³⁺][OH^-]³ …………… (1)

We are given with

Ksp = 6.3 x 10^-38   [Fe³⁺]= 1.0 x 10^-5 M

From this let us calculate [OH^‑] =? Using eq. (1)

6.3 x 10^-38 = (1.0 x 10^-5) [OH^-]³

[OH^‑]³ = (6.3 x 10^-38) / (1.0 x 10^-5)

[OH^‑]³ = (6.3 x 10^-33)

Taking cube roots of both sides,

[OH^‑] = 1.85 x 10^-11 M

From this we can calculate pOH

pOH = -log(1.85 x 10^-11)

pOH = 10.73

Hence,

pH = 14 – 10.73

pH = 3.27

This means at pH 3.27 Fe(OH)₃ will start to precipitate and above pH 3.27 only it will precipitate.

Below pH 3.27 ,for Fe(OH)₃ Solubility product > Ionic product and no precipitation occur.

Hence Fe(OH)₃ will not precipitate at pH = 1.

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