# Conjugate (acid-base theory)

In Brønsted–Lowry acid-base theory, a **conjugate acid-base pair** is two chemical species that differ by exactly one proton (H⁺): the acid is the member that can donate the proton, and the base is the member that can accept it. According to the IUPAC Compendium of Chemical Terminology, when a base \( \ce{B} \) gains a proton to become the Brønsted acid \( \ce{BH+} \), that acid is termed the conjugate acid of \( \ce{B} \), while \( \ce{B} \) in turn serves as the conjugate base of \( \ce{BH+} \).<sup>[1](https://goldbook.iupac.org/terms/view/C01266)</sup> In the reaction of hydrogen chloride with water, chloride ion is the conjugate base of HCl and hydronium ion is the conjugate acid of water.<sup>[2](https://openstax.org/books/organic-chemistry/pages/2-7-acids-and-bases-the-bronsted-lowry-definition)</sup>

| Key fact | Detail |
|---|---|
| Definition | The Brønsted acid \( \ce{BH+} \) formed on protonation of a base \( \ce{B} \) is the conjugate acid of \( \ce{B} \); \( \ce{B} \) is the conjugate base of \( \ce{BH+} \)<sup>[1](https://goldbook.iupac.org/terms/view/C01266)</sup> |
| Charge relation | The conjugate acid always carries one unit of positive charge more than the base, but the absolute charges of the species are immaterial to the definition<sup>[1](https://goldbook.iupac.org/terms/view/C01266)</sup> |
| Canonical example | HCl and Cl⁻ constitute a conjugate acid–base pair<sup>[1](https://goldbook.iupac.org/terms/view/C01266)</sup> |
| Strength relation | \( K_{\mathrm{a}} \times K_{\mathrm{b}} = K_{\mathrm{w}} = 1.0 \times 10^{-14} \) at 25 °C, so the stronger the acid or base, the weaker its conjugate partner<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup> |
| Logarithmic form | \( pK_{\mathrm{a}} + pK_{\mathrm{b}} = pK_{\mathrm{w}} = 14.00 \) at 25 °C<sup>[4](https://chem.libretexts.org/Courses/University_of_Missouri/MU%3A__1330H_%28Keller%29/16%3A_AcidBase_Equilibria/16.05%3A_Strong_Acids_and_Bases)</sup> |
| Leveling effect | Hydronium is the strongest acid that can exist in water; HCl, HBr, and HI are strong acids in water but weak acids in ethanol, with strength increasing HCl < HBr < HI<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup> |

## Definition and scope

The pair concept comes from the Brønsted–Lowry theory, which defines acids as substances that donate a proton and bases as substances that accept one.<sup>[5](https://media.iupac.org/didac/Didac%20Eng/Didac01/Content/H01.htm)</sup> Every acid-base reaction therefore creates two conjugate pairs: the acid becomes its conjugate base after losing a proton, and the base becomes its conjugate acid after gaining one.<sup>[2](https://openstax.org/books/organic-chemistry/pages/2-7-acids-and-bases-the-bronsted-lowry-definition)</sup> The charge rule is relative, not absolute: the conjugate acid carries one more positive charge than its base, so a neutral acid such as acetic acid pairs with the anion acetate, while a cationic acid such as ammonium (\( \ce{NH4+} \)) pairs with the neutral molecule ammonia.<sup>[1](https://goldbook.iupac.org/terms/view/C01266)</sup>

The definition also widens the cast of possible acids and bases beyond the Arrhenius scheme: positive and negative ions as well as neutral molecules can be Brønsted acids, and any molecule or ion with at least one pair of nonbonding valence electrons can be a Brønsted base.<sup>[6](https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/conjugat.php)</sup> Water itself sits in both roles, accepting a proton from HCl to give hydronium and donating one to ammonia to give ammonium and hydroxide.<sup>[2](https://openstax.org/books/organic-chemistry/pages/2-7-acids-and-bases-the-bronsted-lowry-definition)</sup>

## Strength of conjugate partners

The strength of an acid or base is quantified by its ionization constant, \( K_{\mathrm{a}} \) or \( K_{\mathrm{b}} \), which measures the extent of the ionization reaction in water.<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup> For any conjugate pair the two constants are tied together by the ion product of water:

\[ K_{\mathrm{a}} \times K_{\mathrm{b}} = K_{\mathrm{w}} = 1.0 \times 10^{-14} \quad (25\,^\circ\mathrm{C}) \]

This relation holds for any base and its conjugate acid, or any acid and its conjugate base.<sup>[7](https://openstax.org/books/chemistry/pages/14-4-hydrolysis-of-salt-solutions)</sup> The consequence is an inverse relation: the stronger the acid, the weaker its conjugate base, and the stronger the base, the weaker its conjugate acid.<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup> A strong acid loses H⁺ easily, which means its conjugate base holds the proton weakly and is a weak base; water, a weak acid, pairs with hydroxide, which holds H⁺ tightly and is a strong base.<sup>[8](https://openstax.org/books/organic-chemistry/pages/2-8-acid-and-base-strength)</sup> In the limit, a strong acid with an immeasurably large \( K_{\mathrm{a}} \) has a conjugate base whose \( K_{\mathrm{b}} \) is essentially zero.<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup>

The relation is also a practical calculation tool, because many handbooks list only \( K_{\mathrm{a}} \) values. The \( K_{\mathrm{b}} \) of the acetate ion, for example, follows from the \( K_{\mathrm{a}} \) of acetic acid: \( K_{\mathrm{b}} = 1.0 \times 10^{-14} / 1.8 \times 10^{-5} = 5.6 \times 10^{-10} \).<sup>[7](https://openstax.org/books/chemistry/pages/14-4-hydrolysis-of-salt-solutions)</sup> Conversely, given \( K_{\mathrm{b}} = 2.17 \times 10^{-11} \) for the nitrite ion, the \( K_{\mathrm{a}} \) of its conjugate acid HNO₂ is \( 4.6 \times 10^{-4} \).<sup>[9](https://openstax.org/books/qu%C3%ADmica-2ed/pages/14-3-fuerza-relativa-de-los-acidos-y-las-bases)</sup>

## Molecular structure and conjugate-base stability

Acidity tracks the stability of the conjugate base: the more stable the base left behind after proton loss, the stronger the acid.<sup>[10](https://chemistry.stackexchange.com/questions/39451/how-to-assess-the-strength-of-the-acid)</sup> [Resonance](https://www.edgepedia.org/resonance) delocalization, inductive electron withdrawal, and the spread of negative charge over a larger atom all stabilize the conjugate base. This explains why the acid strength of the binary hydrogen halides increases HF < HCl < HBr < HI down group 17, as the larger anion disperses charge over a greater area, and why the group 16 hydrides follow H₂O < H₂S < H₂Se < H₂Te.<sup>[11](https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relative-strengths-of-acids-and-bases/)</sup> Across a row, acid strength increases with the electronegativity of the nonmetal, giving CH₄ < NH₃ < H₂O < HF in the second row.<sup>[11](https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relative-strengths-of-acids-and-bases/)</sup>

Some representative pairs, with the \( pK_{\mathrm{a}} \) of the acid and its conjugate base:<sup>[8](https://openstax.org/books/organic-chemistry/pages/2-8-acid-and-base-strength)</sup>

| Acid | pKa | Conjugate base |
|---|---|---|
| Hydrochloric acid, HCl | −7.0 | Chloride ion, Cl⁻ |
| Nitric acid, HNO₃ | −1.3 | Nitrate ion, NO₃⁻ |
| Phosphoric acid, H₃PO₄ | 2.16 | Dihydrogen phosphate ion, H₂PO₄⁻ |
| Acetic acid, CH₃CO₂H | 4.76 | Acetate ion, CH₃CO₂⁻ |
| Hydrocyanic acid, HCN | 9.31 | Cyanide ion, CN⁻ |
| Water, H₂O | 15.74 | Hydroxide ion, HO⁻ |
| Ethanol, CH₃CH₂OH | 16.00 | Ethoxide ion, CH₃CH₂O⁻ |

## The leveling effect

A solvent constrains which conjugate strengths can be observed. In water, the sole acid species present in a solution of any strong acid is \( \ce{H3O+} \), so hydronium is the strongest acid that can exist in water; any stronger acid reacts completely with water to generate hydronium ions. This limit is called the leveling effect.<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup> The same applies on the basic side: species stronger than hydroxide, such as the oxide ion \( \ce{O2-} \) and the amide ion \( \ce{NH2-} \), react completely with water and are leveled to hydroxide.<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup>

To distinguish the strengths of strong acids, they must be dissolved in a solvent less basic than water. In ethanol, the hydrogen halides behave as weak acids and their true ordering appears, with ionization increasing HCl < HBr < HI.<sup>[3](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)</sup> Solvents that reveal such differences are called differentiating solvents; acetonitrile, a very weak base and an exceptionally weak acid, levels acids and bases only slightly.<sup>[12](https://mazams.weebly.com/uploads/4/8/2/6/48260335/solvents_and_solvent_effects_in_organic_chemistry_3rd_ed.pdf)</sup>

## Use in predicting solution behavior

Conjugate strengths predict the pH of salt solutions from neutralization reactions. A strong acid and a strong base give a neutral solution because both conjugate partners produced are of negligible strength. A strong acid with a weak base gives an acidic solution: the conjugate base of the strong acid (\( K_{\mathrm{b}} \approx 0 \)) does not affect pH, while the conjugate acid of the weak base (\( K_{\mathrm{a}} > 0 \)) ionizes to make the solution acidic. A weak acid with a strong base gives a basic solution for the symmetric reason. When a weak acid meets a weak base, the outcome depends on the relative strengths of the two conjugates, which must be compared through tabulated \( K \) values.<sup>[7](https://openstax.org/books/chemistry/pages/14-4-hydrolysis-of-salt-solutions)</sup>

## References

1. [IUPAC Compendium of Chemical Terminology, conjugate acid–base pair (C01266)](https://goldbook.iupac.org/terms/view/C01266)
2. [OpenStax Organic Chemistry, 2.7 Acids and Bases: The Brønsted–Lowry Definition](https://openstax.org/books/organic-chemistry/pages/2-7-acids-and-bases-the-bronsted-lowry-definition)
3. [OpenStax Chemistry 2e, 14.3 Relative Strengths of Acids and Bases](https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases)
4. [Chemistry LibreTexts, 16.5 Strong Acids and Bases](https://chem.libretexts.org/Courses/University_of_Missouri/MU%3A__1330H_%28Keller%29/16%3A_AcidBase_Equilibria/16.05%3A_Strong_Acids_and_Bases)
5. [IUPAC Didac, H01 Conjugate acid-base pairs (Brønsted-Lowry theory)](https://media.iupac.org/didac/Didac%20Eng/Didac01/Content/H01.htm)
6. [Purdue University ChemEd, Acid-Base Pairs, Strength of Acids and Bases, and pH](https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/conjugat.php)
7. [OpenStax Chemistry, 14.4 Hydrolysis of Salt Solutions](https://openstax.org/books/chemistry/pages/14-4-hydrolysis-of-salt-solutions)
8. [OpenStax Organic Chemistry, 2.8 Acid and Base Strength](https://openstax.org/books/organic-chemistry/pages/2-8-acid-and-base-strength)
9. [OpenStax Química 2ed, 14.3 Fuerza relativa de los ácidos y las bases](https://openstax.org/books/qu%C3%ADmica-2ed/pages/14-3-fuerza-relativa-de-los-acidos-y-las-bases)
10. [Chemistry Stack Exchange, How to assess the strength of the acid?](https://chemistry.stackexchange.com/questions/39451/how-to-assess-the-strength-of-the-acid)
11. [Pressbooks (University of Hawaiʻi), Relative Strengths of Acids and Bases](https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relative-strengths-of-acids-and-bases/)
12. [Reichardt, Solvents and Solvent Effects in Organic Chemistry, 3rd ed., front matter](https://mazams.weebly.com/uploads/4/8/2/6/48260335/solvents_and_solvent_effects_in_organic_chemistry_3rd_ed.pdf)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Thermodynamics and equilibrium › Chemical equilibrium › Acid–base equilibrium*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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