Oxides are salts of the #O^(2-)# 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 #M^(2+)#; this would form a salt with phosphoric acid of the form....
#M^(2+)(HPO_4)^(2-)#.....usually we encounter biphosphate salts in aqueous solution. However, the answer they are probably looking for is #M_3(PO_4)_2# (these salts are actually quite rare!).
So can you give the formula for lithium, calcium, and iron(II), and iron(III) phosphates?
