[Serenagreene]

Okay what am I doing wrong???

The Ksp of Iron (III) hydroxide is 8 * 10^-16.  How many ug of iron (III) hydroxide could be dissolved in 250 ml of solution?

Fe(OH)3 <--> Fe + 3 OH

Ksp = [Fe][OH]^3

8 * 10^-16  = X^4

X = 1.68 M

1.68 E^-4 = x/.250
x = 4.2 E^-5 moles * 106.87 g/mol = 4.49 E^-3 g

Teacher answer is 131 ug.

[Borek]

8 * 10^-16  = X^4

Why x⁴? Are concentrations of Fe³⁺ and OH^- identical?

[Serenagreene]

Okay there was one problem.  I forgot that the 3 OH translates to a [3 OH}^3

So Ksp = [Fe][3 OH]^3
8E-16 = 3x^4
x =  1.28E-4 M  (I also dropped off the E-4 last night

1.28E-4 = x/.250

x = .093 moles * 106.87 g/mol = 9.95 g

Still a long way from the teacher's answer of 131 ug.

[Borek]

Where did you get Kso value from?

Your math is still wrong, although you are a step closer.

[OH^-] = 3[Fe³⁺]

[OH^-]³ = (3[Fe³⁺])³ = ?

[Serenagreene]

The Ksp figure came from the teacher's problem. 

[Borek]

it is about 20 orders of magnitude off.