How do I balance the following equation by the half-reaction method (it takes place in basic solution)?

Question

Fe(OH)2(s) + O2(g) = Fe(OH)3(s)
the answer is 4Fe(OH)2(s) + 2H20(l) + O2(g) = 4Fe(OH)3(s) but how do I work it out?

Answer 1

Here is a set of rules for balancing half reactions in both acid and basic solutions.
http://www.chemteam.info/Redox/Redox.html