Thu November 03, 2011 By:


Expert Reply
Sat November 12, 2011
The two sulfurs bonded to three oxygens and one sulfur, for an oxidation of +3; but they also carry a +2 formal charge, so their oxidation state is +5.
On the other hand, the two sulfurs  bonded only to sulfur, and have no formal charge; their oxidation state is 0 (zero). The total oxidation states of the four sulfur atoms is 5 + 5 + 0 + 0 = 10, or an average of +2.5.
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