The correct order of following 3d metal oxides, according to their oxidation numbers is :
(a) CrO₃, (b) Fe₂O₃, (c) MnO₂, (d) V₂O₅, (e) Cu₂O

Solution

Chemistry ma’am · 3 steps

Give each oxygen −2, solve for one metal atom, then line up the five numbers from high to low.

1. Set up the rule. In each oxide oxygen is -2 and the compound is neutral. So for MₓO_y, the metal's oxidation state is x × n = 2y, which gives n = 2y/x.

   Oxide	Working	Oxidation state of metal
   (a) CrO₃	n = (2 × 3)/1	+6
   (d) V₂O₅	n = (2 × 5)/2	+5
   (c) MnO₂	n = (2 × 2)/1	+4
   (b) Fe₂O₃	n = (2 × 3)/2	+3
   (e) Cu₂O	n = (2 × 1)/2	+1

2. Arrange them. From highest to lowest: +6 > +5 > +4 > +3 > +1. That is CrO₃ > V₂O₅ > MnO₂ > Fe₂O₃ > Cu₂O.
3. So. In the question's labels this is (a) > (d) > (c) > (b) > (e), which is option (c).

The idea to keep. For an oxide MₓO_y, the metal's oxidation state is 2y/x. Always divide by the number of metal atoms. Do not judge by the number of oxygens or by the group number.

Answer from the NTA final answer key. Solution by PrepWiser.