alleryStart learning →What is the oxidation number of sulfur in the sulfite ion, SO3 2-?
APlus six
BPlus four
CMinus two
DPlus two
Answer & Solution
Correct answer: B. Plus four
1. Oxygen takes its usual value of -2 in this ion.
2. Three oxygen atoms contribute 3 times -2, which is -6.
3. The sum of oxidation numbers must equal the charge on the ion, which is -2.
4. So -2 = (-6) + x for the sulfur atom.
5. Rearranging gives x = -2 minus (-6), that is +4.
6. The value +6 belongs to sulfur in sulfate, SO4 2-, which has one more oxygen atom.
7. Setting the sum to zero instead of -2 is the trap that produces +6 here.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.2 Classifying Chemical Reactions_
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