[MakoEyes]

Give the oxidation number of phosphorus in each of the following compounds or ions:
(a) Ca3P2 (b) H3PO3 (c) P2O74– (d) P4O8 (e) Ca(H2PO4)2


How can you do a problem of this nature?

[Borek]

Assigning oxidation numbers.

[MakoEyes]

Thus for these compounds I'd be following the rule that states:

Last rule says that the charge of the ion or molecule equals sum of oxidation numbers of all atoms.

But then they give an example that only counts the number of Oxygen atoms for oxidation? This is contradictory.

So would this mean...
(a) Ca3P2 = 12?
(b) H3PO3 = 6 or 12?
(c) P2O74– = How do I approach this one?
(d) P4O8 = 16 or 28?
(e) Ca(H2PO4)2 = 16 or 28?


I'm still a bit confused, thanks for that link though, it was helpful.

[Borek]

Oxidation number is assigned to INDIVIDUAL atom, not molecule or ion as a whole.

[MakoEyes]

Ah ok I see. Let me try to rework it:


(a) Ca3P2 = -3
(b) H3PO3 = +3
(c) P2O74– = Still unsure about this one. Looking for a proof?
(d) P4O8 = +4
(e) Ca(H2PO4)2 = Also unsure about this one, any additional help would be appreciated.

[Borek]

(c) P2O74– = Still unsure about this one.

2*ON_P + 7*(-2) = -4.

Straight application of the rules. Oxygen is -2, ion charge is -4. ON_P is oxidation number of P in the ion.