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What is the oxidation number of Chromium in Cr2O74-?

There are seven oxygen atoms, so the total charge is −2⋅7=−14 . So, two chromium atoms have a total oxidation number of +12 . Since one is not more electronegative than the other, we can divide the number by two to get one chromium's oxidation number. So, a chromium atom here has an oxidation number of +6 .

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the correct answer is a

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.