Net charge for phosphate and phosphite ion

[Jigneshbharati]

TL;DR Summary

Why is the net charge the same (-3) for phosphate (PO4) and (PO3)?

Why is the net charge the same (-3) for phosphate and phosphite ion?
PO4-3 and PO3 is also -3.
This is nor a homework question. I am 49 and love chemistry and helping my son with his studies.
Thank you in advance.

 

[Mayhem]

Different oxidation states. By convention (and perhaps necessity), oxygen has a -2 oxidation state with notable exceptions being O₂ (where it is 0) and H₂O₂ where it is -1.

Can you deduce the oxidation state of P in these two compounds and justify the identical net charge? Also remember that phosphite has an H on it! (+1 oxidation state typically)

 

[symbolipoint]

Surprising to find Phosphite ion does have a Hydrogen in it.
HPO₂^-2

Basic facts are, O has charge -2, and H has a charge of +1.
You were interested in what charge goes with the P.
The ion's charge is -2.

1(+1)+1(x)+2(-2)=-2

1+x-4=-2

x=-2-1+4

x=+1

 

[snorkack]

Surprising to find Phosphite ion does have a Hydrogen in it.
HPO₂^-2

It is surprising and somewhat unusual but it is one of the relatively notable facts about chemistry of phosphorus. Likewise, phosphorus has hypophosphite H₃PO₂ which is importantly a monobasic acid.
But the underlying logic is that while the low coordination numbers of P (and S) prevail in lower halides, they are unstable for O and sometimes F compounds. P prefers to have either P-H bonds or else "saturate" P-O bonds by reaching the coordination number 4. Both phosphites and hypophosphites will dismute on heating - to PH₃ and H₃PO₄.
This requires exchanging hydrogens between P atoms. At room temperature the compositions H₃PO₃ and H₃PO₂ are stable... but the molecules rearrange to add the P=O bond:
P(OH)₃>O=P-H(OH)₂
goes to practically completion and is not reversed by bases.
Realized a better comparison. You do not have "carbonous acid" C(OH)₂ either. You do have acid H₂CO₂... but it is actually monobasic HCOOH, the formic acid.