Oxides are salts of the O2− ion.... Since these salts are necessarily neutral, we add the appropriate number of metal cations to give a NEUTRAL charge.....
So for MO we had a salt of M2+; this would form a salt with phosphoric acid of the form....
M2+(HPO4)2−.....usually we encounter biphosphate salts in aqueous solution. However, the answer they are probably looking for is M3(PO4)2 (these salts are actually quite rare!).
So can you give the formula for lithium, calcium, and iron(II), and iron(III) phosphates?
