Determine the oxidation numbers of various elements for the following chemicals: (NH4)3PO4 Ba3P2 CO2 NaOH Al2O3 P2O5 Na2O2 FeO Fe2O3

The oxidation number of any element in a given compound can be determined by using some simple rules. For example, the sum of oxidation numbers of all the elements is equal to the charge over a given compound. So it would be 0 for O2 (oxygen gas) and -1 for OH- ion. Similarly, some elements always have the same oxidation number, for example hydrogen has an oxidation number of +1 (except for H2 gas) and oxygen has an oxidation number of -2 (except for O2 gas), etc.

Let us look at couple of the compounds mentioned in the question.

NaOH: 

oxidation number of Na + oxidation number of oxygen + oxidation number of hydrogen = 0 

which means, oxidation number of Na + (-2) + (+1) = 0

or, oxidation number of Na = -1 +2 = +1

P2O5: 

2x oxidation number of P + 5x oxidation number of oxygen = 0

or, 2 x oxidation number of P + 5 x (-2) = 0

or, oxidation number of P = (5 x 2)/2 = +5.

You can determine the oxidation numbers of different elements for the other cases in the same way.

Hope this helps.