Which species of Fe are considered dissolved Fe?

[1question]

Homework Statement

The question tells me that all dissolved iron species will have an activity of 0.1M. So is Fe(OH)₄^2- considered a dissolved species of Fe, and if so, it will have an activity of 0.1M?

Homework Equations

N/A

The Attempt at a Solution

I want to say that it is not considered as such, but the fact that it is an ion troubles me.

 

[Borek]

The question tells me that all dissolved iron species will have an activity of 0.1M.

Doesn't sound correct to me, can you post entire question?

 

[1question]

It's an assumption so that I can draw a simplified Eh-Ph diagram. It is not meant to be generally correct I believe.

 

[Borek]

Every water solution containing iron ions will also have many equilibria present, with numerous products of Fe(II) and Fe(III) reactions with water. You can assume activity of the main (dominating) form present in the solution to be 0.1 M, or you can assume sum of all forms present to be 0.1 M (it will be just a mass balance then), but in neither case activities of other forms will be 0.1 M.

Chances are this is just a lousy wording and there is a viable idea behind the statement, but without the context I can't see it.

 

[1question]

I asked my Prof. It does not have an activity of 0.1M. Thanks for your help Borek.