alleryStart learning →What is the oxidation number of sulfur in Na2SO4?
APlus four
BPlus two
CMinus two
DPlus six
Answer & Solution
Correct answer: D. Plus six
1. Na2SO4 is ionic, so handle the cation and the anion separately.
2. Sodium is a monatomic ion of charge 1+, so its oxidation number is +1.
3. The anion left is sulfate, SO4 2-.
4. Oxygen takes its usual -2, so four oxygen atoms give 4 times -2, which is -8.
5. The sum for the ion must equal its charge: -2 = (-8) + x.
6. So x = -2 minus (-8), which is +6.
7. Answering +4 would be treating the anion as sulfite rather than sulfate.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.2 Classifying Chemical Reactions_
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