Oxidation numbers: Class 11 practice
Rules: elements are 0; O is usually −2 (−1 in peroxides); H is +1 with non-metals; the sum of oxidation numbers equals the overall charge. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Valency
- Ion charges
The idea behind oxidation numbers
Rules: elements are 0; O is usually −2 (−1 in peroxides); H is +1 with non-metals; the sum of oxidation numbers equals the overall charge. Solve for the unknown atom.
Step-by-step method
- Assign known oxidation numbers (O, H, alkali metals).
- Set the sum equal to the charge.
- Solve for the unknown.
Worked example 1 (Basic)
Find the oxidation number of Fe in Fe₃O₄.
- Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge
- Fe = +8/3
- Answer: +8/3
Worked example 2 (Standard)
Find the oxidation number of Cr in K₂Cr₂O₇.
- Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge
- Cr = +6
- Answer: +6
Worked example 3 (Challenge)
Find the oxidation number of Mn in KMnO₄.
- Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge
- Mn = +7
- Answer: +7
Common mistakes and misconceptions
- Forgetting the ion's charge
- Treating O in H₂O₂ as −2
Practice questions
- (Basic) Find the oxidation number of P in PO₄³⁻.
- (Basic) Find the oxidation number of Mn in MnO₂.
- (Standard) Find the oxidation number of N in NO₂.
- (Standard) Find the oxidation number of N in HNO₃.
- (Challenge) Find the oxidation number of C in CO₃²⁻.
- (Challenge) Find the oxidation number of S in S₂O₃²⁻.
Answer key with full working
- Q1: Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge → P = +5 → Answer: +5
- Q2: Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge → Mn = +4 → Answer: +4
- Q3: Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge → N = +4 → Answer: +4
- Q4: Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge → N = +5 → Answer: +5
- Q5: Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge → C = +4 → Answer: +4
- Q6: Use O = −2 (−1 in peroxide), H = +1, alkali metals = +1, and the overall charge → S = +2 → Answer: +2
Syllabus note
This skill is listed under "Redox Reactions" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is oxidation numbers in?
"Redox Reactions", Class 11 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.