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

2 Answers
Rory K. · Stefan V.
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#.

anor277
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....

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