Determine the oxidation state of chromium in potassium dichromate, K2Cr2O7.

• A. +3
• B. +4
• C. 0
• D. +6

Answer: (D)

Oxidation states are assigned so that the sum matches the overall charge, using the usual values for common elements: potassium is almost always +1 and oxygen is almost always -2. In potassium dichromate, there are two potassium ions contributing +2 and seven oxygens contributing -14. The dichromate unit contains two chromium atoms whose total oxidation state must balance the remaining charge to fit the overall neutral salt, or, equivalently for the anion Cr2O7^2-, balance to -2.

Let the oxidation state of chromium be x. For the salt as a whole: 2(+1) + 2x + 7(-2) = 0. This gives 2 + 2x - 14 = 0, so 2x = 12 and x = +6. Therefore chromium is in the +6 oxidation state.