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What is the oxidation number of SO3?
There are generally two possible answer to the question:
The Oxidation states in SO3(g) are: Sulfur (+6) & Oxygen (-2), because SO3(g) has no charge.
However in ##(SO_3)_2## - (aq) the Oxidation states are: Sulfur (+4) & Oxygen (-2).
Don't get the two confused, they may both be written without the charge, but if SO3 is (aq) it will have a charge of -2.