Welcome to Chembalancer!    
2 Cu + 1 O2 2 Cu-0
Cu Cu-0
Cu O-O Cu-0


This is a copy of a little online game at http://www.dun.org/sulan/chembalancer to teach balancing equations. You type numbers in front of each molecule until there are the same number of atoms on both sides of the equation, then click the 'Balanced' button.