[loganro]

I am having trouble understanding what oxidation numbers to give to certain things.  For example, HPO₃ is a problem that I have (I know that the answer is +5) ,and I can't seem to understand why that is the answer.  H₃PO₂ oxidation number is +1.   The question is asking to indicate the oxidation number of phosphourus.  Why does the oxidation number change? How do you know when to change it? I have looked on the "List of Oxidation Rules," but I can't seem to understand it.  The way I understand the list, "P" would get the same oxidation number with all elements.

[agrobert]

In general inorganic compounds oxygen has a oxidation state of -2 and hydrogen +1 (unless you have a hydride -1).  In your case phosphorous changes oxidation state.  Calculate the charge without the phosphorous and then balance it.  This number is your oxidation state.  For example, HPO₃ 3 Oxygen (3*-2) = -6 and 1 Hydrogen (+1) -6+1=-5 so phosphorous is +5.

[loganro]

Thanks for the help.  IF₇;  I'm pretty sure that the oxidation number for F₇ is is -7, but the problem is wanting to know the oxidation number for I.  Would the oxidation number for I be +6?

[agrobert]

The oxidation number for F- is (-1) and there are seven. So IF₇ has oxidation of __ for I to balance __?  When you first start learning oxidation states don't second guess yourself, you are usually correct.  Remember to always balance a neutral molecule and in general H(+1), O(-2) and halogens in there normal OS (-1).  Don't be afraid to assign high oxidation states.  Think about HClO₄...the chlorine has the same oxidation state as I in IF₇.