General

[quote=jaesang0]for each reaction, decompose each species on the left and right into two or more smaller reactions. furthermore, take note of any changes in the oxidation number of each species
for example, in the first reaction As changes from an oxidation number of 0 to +5 (assuming As4 exists naturally in nature, like O2 or N2).
this site does a nice job of explaining it. i don't want to give away the answer so i'll let you work through it
http://chemistry.about.com/od/generalchemistry/ss/redoxbal.htm[/quote]

Thanks for the tips, but our teacher said that we can't use the half-reaction method to solve it ... uhhhg I don't get this xD