What is a polyprotic acid? What does it mean to be amphiprotic? Can the conjugate base of a polyprotic acid gain a proton? Can the conjugate base of a polyprotic acid lose a proton? Is the conjugate base of a polyprotic acid always amphiprotic?

A polyprotic acid is an acid that can donate multiple protons (H+) per molecule. It typically has more than one ionizable hydrogen atom. For example, sulfuric acid (H2SO4) is a polyprotic acid because it can donate two protons in separate steps:

H2SO4 ⇌ H+ + HSO4-
HSO4- ⇌ H+ + SO42-

Being amphiprotic means a substance can act as both an acid (donate a proton) and a base (accept a proton). For example, water (H2O) is amphiprotic because it can donate a proton to form a hydroxide ion (OH-) or accept a proton to form a hydronium ion (H3O+):

H2O ⇌ H+ + OH-
H2O + H+ ⇌ H3O+

The conjugate base of a polyprotic acid can gain a proton to become the acid. In the example of sulfuric acid, the conjugate base HSO4- can gain a proton to reform the acid:

HSO4- + H+ ⇌ H2SO4

Similarly, the conjugate base of a polyprotic acid can lose a proton to become a different conjugate base. For sulfuric acid, the conjugate base SO42- can lose a proton to form HSO4-:

SO42- ⇌ H+ + HSO4-

Not all conjugate bases of polyprotic acids are amphiprotic. In the case of sulfuric acid, the conjugate base SO42- is not amphiprotic because it cannot accept a proton. However, the conjugate base HSO4- is amphiprotic because it can both accept and donate a proton. So, the amphiprotic nature of the conjugate base depends on the specific acid-base properties of each species involved.