Sodium hydroxide and sodium dihydrogen phosphate?

Answer 1

The monodeprotonated phosphoric acid is still an acid, and so is the dideprotonated one, so you would get a mixture of different ions, but if you make the solution sufficiently basic you will get tri sodium phosphate. Na3PO4, which is moderate basic base (just guessing).

But at PHs found it living tissue you would get a buffer from that ions and a mixture of all three ions, most of the first one H2(PO4)-, less of the second H(PO4)2- and almost none of the last (PO4)3-, there will even be some of the fully protonated acid H3(PO4). In what concentrations they are present in is of course based on volume of water, amount of compound added and the pH value.

Look up the pKa values for each of the 3 protons and you can calculate the exact composition at a given pH and also calculate to what extend the buffer prevent pH change.

Answer 2

When sodium hydroxide reacts with sodium dihydrogen phosphate, the products are sodium phosphate and water. The balanced chemical equation for this reaction is 2 NaOH + NaH2PO4 → Na3PO4 + 2 H2O. It is a double displacement reaction.