Fundamentals According to the Bronsted-lowery theory, a conjugate acid is a species which is formed by reaction of proton with a base. a conjugate base is a species which is formed when acid loses a proton. The reaction representing the formation of conjugate acid and conjugate base is as follow: 
Acid donates hydrogen ion and base is the acceptor of proton. The conjugate acid base pair differs by one proton. Conjugate acid is formed when a hydrogen ion is added to the base. Whereas, the conjugate base is formed when an acid donates its proton (hydrogen ion). Therefore, the base, hydrogen phosphate ion reacts with a proton to form a conjugate acid.
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Maxie S. answered 04/14/20 Tutor
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When a bronsted-lowry acid is in equilibrium, it will donate a proton (as acids do) in the form of a hydrogen ion, and will therefore become a base (willing to accept a proton). The resulting base is called the conjugate base. The same is true for bases– when in equilibrium, it will accept a proton, and become a conjugate acid.
Here are the equilibrium expressions:
HPO42- + H2O ↔ PO43- + H3O+ (conjugate base)
HPO42- + H2O ↔ H2PO41- + OH1- (conjugate acid)
FAQ
What is the conjugate acid of HPO2 2 −?
Is HPO42 and H2PO4 a conjugate acid base pair?
