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What is the oxidation number of nitrogen in KNO3?
APlus five
BPlus three
CMinus three
DPlus one
Answer & Solution
Correct answer: A. Plus five
1. Potassium is a group 1 cation of charge 1+, so its oxidation number is +1.
2. Oxygen takes the usual value of -2.
3. Three oxygen atoms contribute 3 times -2, which is -6.
4. The compound is neutral, so the whole sum must be zero.
5. That gives 0 = (+1) + x + (-6) for the nitrogen value x.
6. Solving, x = +5.
7. Answering -3 is the value nitrogen takes in ammonium, where hydrogen rather than oxygen is the partner.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.2 Classifying Chemical Reactions_
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