Conjugate acid

In the Brønsted-Lowry (protonic) theory of acids and bases, a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. The base produced, X^-, is called the conjugate base.

Acid Base
HCL Hydrochloric acid Cl- Chlorine ion
H₂SO₄ Sulfuric acid HSO₄^- Hydrogen sulfate ion
HNO₃ Nitric acid NO₃^- Nitrate ion
H₃O⁺ Hydronium ion H₂O Water
HSO₄^- Hydrogen sulfate ion SO₄^2- Sulfate ion
H₃PO₄ Phosphoric acid H₂PO₄^- Dihydrogen phosphate ion
HC₂H₃O₂ Acetic acid C₂H₃O₂^- Acetate ion
H₂CO₃ Carbonic acid HCO₃^- Hydrogen carbonate ion
H₂S Hydrosulfuric acid HS^-
H₂PO₄^- Dihydrogen phosphate ion HPO₄⁺ Hydrogen phosphate ion
NH₄⁺ Ammonium ion NH₃ Ammonia
HCO₃^- Hydrogen carbonate ion CO₃^2- Carbonate ion
HPO₄^2- Hydrogen phosphate ion PO₄^3- Phosphate ion
H₂O Water OH^- Hydroxide ion

Strength of Conjugate Acid-Base Pairs. Acid strength decreases and base strength increases down the table.

Acid	Base
HCL Hydrochloric acid	Cl- Chlorine ion
H₂SO₄ Sulfuric acid	HSO₄^- Hydrogen sulfate ion
HNO₃ Nitric acid	NO₃^- Nitrate ion
H₃O⁺ Hydronium ion	H₂O Water
HSO₄^- Hydrogen sulfate ion	SO₄^2- Sulfate ion
H₃PO₄ Phosphoric acid	H₂PO₄^- Dihydrogen phosphate ion
HC₂H₃O₂ Acetic acid	C₂H₃O₂^- Acetate ion
H₂CO₃ Carbonic acid	HCO₃^- Hydrogen carbonate ion
H₂S Hydrosulfuric acid	HS^-
H₂PO₄^- Dihydrogen phosphate ion	HPO₄⁺ Hydrogen phosphate ion
NH₄⁺ Ammonium ion	NH₃ Ammonia
HCO₃^- Hydrogen carbonate ion	CO₃^2- Carbonate ion
HPO₄^2- Hydrogen phosphate ion	PO₄^3- Phosphate ion
H₂O Water	OH^- Hydroxide ion