Question

What salt would phosphoric acid form with a metal whose oxide is `M_2O_3`?

Answer 1

`"MPO"_4`

Explanation:

Based on `"M"_2"O"_3`, `"M"` has a valency of `3`. The phosphate group in phosphoric acid `("H"_3"PO"_4)` has a charge of `3-`.

So for each `"PO"_4^(3-)` we need one `"M"^(3+)` to give a neutral salt. i.e `"MPO"_4`.

Answer 2

Well, clearly we gots an `M^(3+)` cation.....to give `M^(3+)PO_4^(3-)`...

Explanation:

We gots `M_2O_3`. The typical oxidation state of oxygen in an oxide (the which we clearly got) is `-II`...electrical neutrality demands that we have `M^(3+)`...i.e. `2xx(+3)+3xx(-2)=0`..as required for a neutral salt. And so its phosphate is formulated as `M^(3+)PO_4^(3-)`

On the other hand (if you are an undergrad), phosphoric acid is ONLY a diacid in water...tritation with sodium hydroxide yields a stoichiometric endpoint at `Na_2^(+)HPO_4^(2-)`...

And so we might have a biphosphate species of the form `M_2(HPO_4)_3`...but given the boundary conditions of the problem, clearly the answer is the former....