CHEM 121 Answers to Oxidation Number Part I Slide 9 a) SO42− Assign -2 to oxygen, and S ends up with +6. b) NH4+ Assign +1 to H and N ends up with −3 c) CoBr3 This is ionic so it forms monatomic ions: Co3+ and Br− So, Co is +3 and Br is −1. d) Fe(NO3)2 This is ionic so it forms Fe2+and NO3−. Fe is 2+. YOU CANNOT GIVE AN OXIDATION FOR NITRATE AS SUCH. Remember oxidation number is for EACH atom, not for the whole ion. So, for nitrate (NO3−), assign −2 for O and N is +5. NOTE ALSO THERE IS NO REASON TO DO (NO3)2. Just do one nitrate (not two). e) CrClO3 This is ionic, breaking up into Cr+ and ClO3−. Cr is +1 and from ClO3−, you assign O as −2 and Cl becomes +5. f) V2(OH)3 V has a variable charge so you have to figure that out from the anion, OH−. Three OH− give a total charge of −3 So V2 would be a total of +3, and EACH V would be +3/2. From OH−, we assign +1 to H and O becomes −2.