+2
The proper name for ##CuSO_4## is copper(II) sulphate – the Roman numerals denote the oxidation state for any atom which has variable oxidation states, such as transition metals.
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 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.
