1. The problem statement, all variables and given/known data The question tells me that all dissolved iron species will have an activity of 0.1M. So is Fe(OH)42- considered a dissolved species of Fe, and if so, it will have an activity of 0.1M? 2. Relevant equations N/A 3. 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.