Conjugate Base Calculators

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A conjugate base is the species that remains after an acid donates a proton (H⁺). In Brønsted-Lowry acid-base theory, every acid has a conjugate base — they differ by exactly one proton. Conjugate base strength is inversely related to the strength of the parent acid: strong acids have weak conjugate bases, and weak acids have relatively strong conjugate bases. Understanding conjugate pairs is essential for acid-base equilibria, buffer chemistry, neutralization reactions, and amino acid biochemistry.

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What Is a Conjugate Base?

In Brønsted-Lowry theory, when an acid (HA) donates a proton (H⁺) to a base, the acid loses one proton and becomes its conjugate base (A⁻):

HA ⇌ H⁺ + A⁻

The conjugate base A⁻ and parent acid HA form a conjugate acid-base pair — they differ by exactly one H⁺. The reverse reaction (A⁻ accepting H⁺ to regenerate HA) is the conjugate base acting as a Brønsted-Lowry base.

Common Conjugate Acid-Base Pairs

Acid (HA)Conjugate Base (A⁻)
HCl (strong acid)Cl⁻ (extremely weak base)
H₂SO₄ (strong)HSO₄⁻ (weak acid/base)
CH₃COOH (acetic acid, pKa 4.76)CH₃COO⁻ (acetate)
H₂CO₃ (carbonic acid, pKa 6.35)HCO₃⁻ (bicarbonate)
NH₄⁺ (ammonium, pKa 9.25)NH₃ (ammonia)
H₂O (pKa ~15.7)OH⁻ (hydroxide)

Relationship Between Acid Strength and Conjugate Base Strength

There is an inverse relationship between acid strength and conjugate base strength:

  • Strong acid → very weak conjugate base: HCl fully dissociates; Cl⁻ has essentially no tendency to accept H⁺
  • Weak acid → relatively stronger conjugate base: Acetic acid (pKa 4.76) has acetate (Kb = Kw/Ka) as a significant base

Quantitatively: Ka × Kb = Kw and pKa + pKb = 14 for a conjugate pair at 25°C.

Conjugate Bases in Buffer Solutions

Buffer solutions work by containing both an acid and its conjugate base in significant concentrations. The conjugate base absorbs H⁺ added to the solution (A⁻ + H⁺ → HA), while the acid absorbs OH⁻ (HA + OH⁻ → A⁻ + H₂O). The Henderson-Hasselbalch equation gives the pH as a function of the ratio of conjugate base to acid: pH = pKa + log([A⁻]/[HA]).

Polyprotic Acids and Multiple Conjugate Pairs

Polyprotic acids like H₃PO₄ have multiple conjugate pairs:

  • H₃PO₄ / H₂PO₄⁻ (pKa 2.15)
  • H₂PO₄⁻ / HPO₄²⁻ (pKa 7.20)
  • HPO₄²⁻ / PO₄³⁻ (pKa 12.35)

Each step removes one proton, forming the next conjugate base in the series.

Glossary

Conjugate Base
The species formed when an acid donates a proton. A conjugate base differs from its parent acid by one fewer H⁺ and one more negative charge. A⁻ is the conjugate base of acid HA.
Conjugate Acid-Base Pair
Two species that differ by exactly one proton — an acid (HA) and its conjugate base (A⁻). Ka of the acid × Kb of the conjugate base = Kw = 10⁻¹⁴ at 25°C.
Amphoteric
Capable of acting as both an acid and a base depending on conditions. Water is the classic amphoteric substance. Amino acids and certain anions (like HCO₃⁻) are also amphoteric.

Frequently Asked Questions

Remove one H⁺ (proton) from the acid formula — the resulting species is the conjugate base. For acetic acid (CH₃COOH): remove H⁺ → acetate (CH₃COO⁻). For ammonium (NH₄⁺): remove H⁺ → ammonia (NH₃). For water (H₂O): remove H⁺ → hydroxide (OH⁻). The conjugate base always has one fewer proton and one more negative charge than the parent acid.

Yes. Strong acids dissociate completely because their conjugate bases have essentially no tendency to accept H⁺. For example, HCl dissociates fully, giving Cl⁻ which barely acts as a base at all. The stronger the acid (larger Ka), the weaker its conjugate base (smaller Kb), since Ka × Kb = Kw = 10⁻¹⁴ at 25°C.

In a buffer, the conjugate base (A⁻) reacts with added H⁺ to form the parent acid (HA): A⁻ + H⁺ → HA. This consumes the added H⁺ and prevents a large pH drop. The parent acid simultaneously reacts with added OH⁻: HA + OH⁻ → A⁻ + H₂O. The combination of acid and its conjugate base allows the buffer to resist pH changes in both directions.

Yes — water is amphoteric. As an acid, water donates H⁺ to form its conjugate base OH⁻ (hydroxide). As a base, water accepts H⁺ to form its conjugate acid H₃O⁺ (hydronium). This dual ability is central to the autoionization of water: 2H₂O ⇌ H₃O⁺ + OH⁻, with Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C.