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

2 Answers
Jan 13, 2018

"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.

Jan 13, 2018

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....