Oxidation Number Rules - UNSOLVED PRACTICE SET

Q7. State the rules for assigning oxidation numbers to elements in compounds. List at least four important rules.

Q8. Calculate the oxidation number of the underlined element in each of the following:

(a) KMnO₄

(b) Na₂S₂O₃

(c) HNO₃

(d) Fe₃O₄

Q9. Explain why the oxidation number of oxygen is –2 in most compounds but –1 in peroxides. Give one example of each.

Q10. Calculate the oxidation number of carbon in:

(a) CH₄

(b) CH₃OH

(c) HCHO

(d) HCOOH

(e) CO₂

What trend do you observe as carbon becomes more oxidised?

Q11. In the compound Fe₃O₄, iron appears to have a fractional oxidation number. Explain how this is possible and what it means at the structural level.

Q12. A student assigns an oxidation number of +7 to chlorine in HClO₃. Identify the error and calculate the correct oxidation number.

Q13. (a) State and explain the rules for assigning oxidation numbers with examples.

(b) Calculate the oxidation number of the underlined elements in:

(i) K₂CrO₄

(ii) Na₂S₄O₆

(iii) H₃PO₄

(iv) OF₂

(v) KO₂

(c) Explain why the oxidation number of an element can be zero, positive, or negative, but never fractional in a simple ionic compound.

Q14. (a) Define oxidation number. How does it differ from valency?

(b) Calculate the oxidation number of:

(i) Mn in KMnO₄, MnO₂, Mn₂O₃, and K₂MnO₄

(ii) S in H₂S, SO₂, H₂SO₃, H₂SO₄, and Na₂S₂O₃

(c) Arrange the sulphur compounds in part (b)(ii) in order of increasing oxidation state of sulphur. What does this trend tell you about the relative oxidising/reducing nature of these compounds?

Q15. (a) Explain why the oxidation number of oxygen is:

(i) –2 in H₂O

(ii) –1 in H₂O₂

(iii) –½ in KO₂ (superoxide)

(iv) +2 in OF₂

(b) Calculate the average oxidation number of iron in Fe₃O₄. Explain why this is an average value and describe the actual oxidation states of iron in this compound.

(c) In the Indian context, rust (Fe₂O₃·nH₂O) forms on iron objects exposed to moist air. Calculate the oxidation number of iron in rust and explain why iron gets oxidised.

Q16. Calculate the oxidation number of the specified element in each of the following compounds:

(a) Cr in K₂Cr₂O₇ and CrO₂Cl₂

(b) P in H₃PO₂, H₃PO₃, and H₃PO₄

(c) N in NH₃, N₂H₄, NH₂OH, N₂O, NO, NO₂, and HNO₃

(d) Cl in Cl₂O, Cl₂O₃, ClO₂, Cl₂O₅, Cl₂O₆, and Cl₂O₇

For each series, plot (mentally or on paper) how the oxidation number changes and identify the most oxidised and most reduced forms.

Q17. A student is given a set of compounds and asked to determine oxidation numbers:

(a) For the compound Na₂[Fe(CN)₆]:

(i) Calculate the oxidation number of Fe

(ii) Calculate the oxidation number of C in the cyanide ligand

(iii) Verify that the sum of oxidation numbers equals the charge on the complex ion

(b) For the compound [Co(NH₃)₆]Cl₃:

(i) Calculate the oxidation number of Co

(ii) Explain why NH₃ is considered a neutral ligand

(iii) Calculate the oxidation number of N in NH₃ (within the complex)

(c) For the organic compound CH₃CHO (acetaldehyde):

(i) Calculate the oxidation number of each carbon atom separately

(ii) Explain why the two carbon atoms have different oxidation numbers

(iii) Calculate the average oxidation number of carbon in this compound

Q18. In Indian agriculture and industry, understanding oxidation states is crucial:

(a) Urea, CO(NH₂)₂, is the most widely used nitrogen fertiliser in India. Calculate the oxidation number of:

(i) Carbon in urea

(ii) Nitrogen in urea

(iii) Explain why the oxidation number of nitrogen in urea is negative despite nitrogen being more electronegative than carbon

(b) In the contact process for manufacturing sulphuric acid, the oxidation number of sulphur changes during the process:

S → SO₂ → SO₃ → H₂SO₄

Calculate the oxidation number of sulphur in each compound and identify where oxidation occurs.

(c) Bleaching powder, CaOCl₂, is widely used for water purification in India. It contains two chlorine atoms with different oxidation states. Calculate the oxidation number of each chlorine atom and explain how this is possible.

(d) A farmer uses ammonium nitrate (NH₄NO₃) as a fertiliser. Calculate the oxidation number of nitrogen in both the cation and the anion. Explain why this compound is called a 'double salt' in terms of oxidation states.