In: Chemistry
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?
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