I was reading this website (http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt) on calculating pH of amphiprotic salts. My question is why can't you do something similar for diprotic acids? Take for example the diprotic acid H2S. Because you have the following reactions: H+ + HS- ⇔ H2S HS- ⇔ H+ + S2- can't you treat HS- like an amphiprotic substance and calculate the pH the same way you calculated pH for other amphiprotic substances? ————————————————————— I also have another question relating to these topics. In this image, I don't understand how they came to the conclusion that [H3AsO4-] ≈ [HAsO42-]. Further, why is H2AsO42- considered amphiprotic when the value of K-1, that is the equilibrium constant for H2AsO42- + H+ ⇔ H3AsO4, is much higher than that of the reaction where it acts an acid?