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

Jan 13, 2018

${\text{MPO}}_{4}$

#### Explanation:

Based on ${\text{M"_2"O}}_{3}$, $\text{M}$ has a valency of $3$. The phosphate group in phosphoric acid $\left({\text{H"_3"PO}}_{4}\right)$ has a charge of $3 -$.

So for each ${\text{PO}}_{4}^{3 -}$ we need one ${\text{M}}^{3 +}$ to give a neutral salt. i.e ${\text{MPO}}_{4}$.

Jan 13, 2018

Well, clearly we gots an ${M}^{3 +}$ cation.....to give ${M}^{3 +} P {O}_{4}^{3 -}$...

#### Explanation:

We gots ${M}_{2} {O}_{3}$. The typical oxidation state of oxygen in an oxide (the which we clearly got) is $- I I$...electrical neutrality demands that we have ${M}^{3 +}$...i.e. $2 \times \left(+ 3\right) + 3 \times \left(- 2\right) = 0$..as required for a neutral salt. And so its phosphate is formulated as ${M}^{3 +} P {O}_{4}^{3 -}$

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 $N {a}_{2}^{+} H P {O}_{4}^{2 -}$...

And so we might have a biphosphate species of the form ${M}_{2} {\left(H P {O}_{4}\right)}_{3}$...but given the boundary conditions of the problem, clearly the answer is the former....