Question

3. Find the minimum pH values for a solution that is 0.100 M in Fe2+ and 0.100 M in Fe3+ where you expect (a) a ppt for Fe(OH)2 and (b) a ppt for Fe(OH)3

Answer 1

From internet , Ksp (Fe(OH)3 =6*10^-38

                       Ksp (Fe(OH)2) = 2*10^-15

a)

Fe(OH)2    ↔ Fe+2 + 2 OH -

Ksp = (Fe+2) * (OH-)^2

Fe+2   =s , (OH-)2 = (2s)^2

So Ksp = (S)*(2s) ^2   = 4s^3 = 2*10^-15

s = (2*10^-15/4)^1/3 =7.93*10^-6

[OH- ] = 7.93*10^-6

pOH = - log(7.93*10^-6) = + 5 .100

pH = 14 - 5.100 = 8.89

b)  

Fe(OH)3↔ Fe +3 + 3OH -

Ksp = (Fe+3) (OH-)^3

Fe+3 = s , (OH-)^3 = (3s)^3

So Ksp = (s)*(3s)^3 = 6*10^-38

27s^3 = 6*10^-38

so s = (6*10^-38/27)^1/3

        = 1.30*10^-13

pOH = -log(!.30*10^-13) = +12.884

pH = 14-pOH = 14 - 12.884 = 1.1157