The oxidation number of kromium in k2Cr2O7
ever try google? Here's one discussion:
http://www.meritnation.com/ask-answer/question/determine-the-oxidation-number-of-cr-in-k2cr2o7/redox-reactions/1515787
http://www.meritnation.com/ask-answer/question/determine-the-oxidation-number-of-cr-in-k2cr2o7/redox-reactions/1515787