A groundwater contains 1.2 mg/L of iron as Fe3+. At what pH will all but 0.002...

A groundwater contains 1.2 mg/L of iron as Fe3+.

At what pH will all but 0.002 mg/L of Fe3+ precipitate, if the solid formed is ferric hydroxide?

Expert Solution

Ksp of Fe(OH)3 = 6.3*10^-38
Molar mass of Fe = 55.845 gm
remaining concetration of Fe3+   = 0.002mg/L = 2*10^-6 g/L
                =   2*10^-6/55.845 mol/L
                 = 3.58*10^-8 M
Fe(OH)3    ----->   Fe3+   +   3 OH-
                                                           X
Ksp = [Fe3+] [OH-]^3
6.3*10^-38 = 3.58*10^-8   * x^3
x = 1.21*10^-10 M

This is [OH-]
[H+] = 10^-14 / [OH-]
= 10^-14 / (1.21*10^-10)
= 8.3*10^-5 M

pH = -log [H+]
= -log (8.3*10^-5)
= 4.08

queen_honey_blossom answered 3 months ago

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