What is the oxidation state of copper in #"CuSO"_4#?
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There are several ways to work this out. We could start from a knowledge that the sulphate ion has a 2- charge, so the copper ion must have a 2+ charge and hence a +2 oxidation number. We could substitute in the oxidation numbers of the sulphur and oxygen atoms, and apply the rule that in a compound the sum of all the oxidation numbers must come to zero. In this case, the oxygen atoms have -2 oxidation number, and sulphur +6, so (4 x -2) + 6 = -2 so the copper must be +2.