Question #5650b

Sodium and the phosphate anion can actually only form one compound, trisodium phosphate, #"Na"_3"PO"_4#.

The other two compounds you're probably referring to are actually formed with the dihydrogen phosphate, #"H"_2"PO"_4^(-)#, and hydrogen phosphate, #"HPO"_4^(2-)#, anions.

Moreover, sodium can form monophosphates and polyphosphates, in several anhydrous and hydrate forms, so I will assume that you're interested in the anhydrous monophosphates here.

• monosodium phosphate

This compound is formed when one sodium cation, #"Na"^(+)#, combines with one dihydrogen phosphate anion, #"H"_2"PO"_4^(-)#.

The salt dissociates completely in aqueous solution

#"NaH"_2"PO"_text(4(aq]) -> "Na"_text((aq])^(+) + "H"_2"PO"_text(4(aq])^(-)#

• disodium phosphate

This compound is formed when two sodium cations combine with one hydrogen phosphate anion, #"HPO"_4^(2-)#.

#"Na"_2"HPO"_text(4(aq]) -> 2"Na"_text((aq])^(+) + "HPO"_text(4(aq])^(2-)#

• trisodium phosphate

This time, you have three sodium cations and one phosphate anion, #"PO"_4^(3-)#.

#"Na"_3"PO"_text(4(aq]) -> 3"Na"_text((aq])^(+) + "PO"_text(4(aq])^(3-)#