What salt would phosphoric acid form with a metal whose oxide is M_2O_3M2O3?

2 Answers
Jan 13, 2018

"MPO"_4MPO4

Explanation:

Based on "M"_2"O"_3M2O3, "M"M has a valency of 33. The phosphate group in phosphoric acid ("H"_3"PO"_4)(H3PO4) has a charge of 3-3.

So for each "PO"_4^(3-)PO34 we need one "M"^(3+)M3+ to give a neutral salt. i.e "MPO"_4MPO4.

Jan 13, 2018

Well, clearly we gots an M^(3+)M3+ cation.....to give M^(3+)PO_4^(3-)M3+PO34...

Explanation:

We gots M_2O_3M2O3. The typical oxidation state of oxygen in an oxide (the which we clearly got) is -IIII...electrical neutrality demands that we have M^(3+)M3+...i.e. 2xx(+3)+3xx(-2)=02×(+3)+3×(2)=0..as required for a neutral salt. And so its phosphate is formulated as M^(3+)PO_4^(3-)M3+PO34

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-)Na+2HPO24...

And so we might have a biphosphate species of the form M_2(HPO_4)_3M2(HPO4)3...but given the boundary conditions of the problem, clearly the answer is the former....