... but why stop at diprotic acids?

With the phosphate system we have three pKs, and thus two ampholytes H2PO4 and HPO42–.

A glance at the graph allows one to estimate the relative amounts of the two major species at any pH.

The pH of a solution of the acid H3PO4 or of the base PO43– is found fin in the same way as those for a monoprotic acid. These work out to pH of about 1.6 for 0.10M H3PO4, and 13.1 for trisodium phosphate.

pHs of the two ampholytes NaH2PO4 and Na2HPO4 are a bit more complicated to estimate, and require some additional background that is beyond the scope of this lesson.  However, it can be done from the plot.

But bear in mind that at this rather high concentration of 0.10M (typical of phosphate buffer solutions), the pKa's found in tables (and used in this plot) will not be accurate.