Iron (II) ions can be precipitated from the following reaction:
4Fe(OH)+(aq) + 4OH-(aq) + O2(g) + 2H2O(l) --> 4Fe(OH)3(s)
How many grams of oxygen gas will be needed to precipitate the iron contained in 125mL of 0.085M Fe(OH)+ solution?
Just follow the steps in this worked example of a stoichiometry problem.
http://www.jiskha.com/science/chemistry/stoichiometry.html