conjugate base
CH3NH2
PO43–
CH3CO2–
HPO42–
acid
CH3NH3+
HPO42–
CH3CO2H
H2PO4–
Acids can be either cations, or molecules or anions.
Bases are proton (H+) acceptors.Bases can be either molecules or anions.
The formulae of the members of an acid-conjugate base pair differ by one H+.The base is the species that is one unit more negative and has one less hydrogen atom.
The acid is the species that is one unit more positive and has one more hydrogen atom.
Note that because of the above relationship between the formulae of an acid and its conjugate base, one member of an acid-base conjugate pair is always charged!
Amphiprotic species are capable of acting either as an acid or as a base.
Amphiprotic species act as the acid in one acid-base conjugate pair, but as the base in another acid-base conjugate pair. For example, H2O and HPO42– (see above) are amphiprotic species.