Conjugate Pairs - MCAT General Chemistry

Conjugate Pairs

10 min read Updated Mar 26, 2026

Every time a Brønsted-Lowry acid donates a proton, it becomes something new - something that could accept a proton back. That “something new” is its conjugate base. Every acid-base reaction creates two conjugate pairs, and understanding this relationship is the key to predicting the direction of proton transfer.

What Conjugate Pairs Are

When an acid (HA) donates a proton, it becomes its conjugate base (A⁻). When a base (B) accepts a proton, it becomes its conjugate acid (BH⁺).

Example: CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

Pair 1: CH₃COOH (acid) and CH₃COO⁻ (conjugate base) - differ by one H⁺
Pair 2: H₂O (base) and H₃O⁺ (conjugate acid) - differ by one H⁺

Every conjugate pair differs by exactly one proton. To find a conjugate base, remove one H⁺. To find a conjugate acid, add one H⁺.

The Inverse Strength Relationship

This is the single most important rule about conjugate pairs:

A strong acid has a very weak conjugate base. A weak acid has a relatively stronger conjugate base.

The same applies in reverse: a strong base has a very weak conjugate acid.

Acid	Strength	Conjugate Base	Strength
HCl	Strong	Cl⁻	Negligible (will not accept H⁺)
H₂SO₄	Strong	HSO₄⁻	Very weak
CH₃COOH	Weak ( $K_a = 1.8 \times 10^{-5}$ )	CH₃COO⁻	Weak but functional
HCN	Very weak ( $K_a = 6.2 \times 10^{-10}$ )	CN⁻	Relatively strong
H₂O	Very weak	OH⁻	Strong

Diagram of water autoionization showing one water molecule donating a proton to another, forming a hydronium ion H3O+ and a hydroxide ion OH-, illustrating conjugate acid-base pairs — Water's autoionization demonstrates conjugate pairs perfectly: one H₂O acts as an acid (donating H⁺ to become OH⁻) while another acts as a base (accepting H⁺ to become H₃O⁺). Each species differs from its conjugate by exactly one proton. *Credit: Wikimedia Commons, Public Domain*

Predicting the Direction of Proton Transfer

In any acid-base equilibrium, the proton transfers from the stronger acid to the stronger base. The equilibrium favors the side with the weaker acid and weaker base.

Rule: Equilibrium favors the formation of the weaker acid-base pair.

Example: Will HF donate a proton to CN⁻?

HF has $K_a = 6.8 \times 10^{-4}$ (weak acid)
HCN has $K_a = 6.2 \times 10^{-10}$ (much weaker acid)

Since HF is the stronger acid and CN⁻ is the stronger base, the proton transfers from HF to CN⁻. The equilibrium lies to the right: HF + CN⁻ ⇌ F⁻ + HCN.

Identifying Conjugate Pairs in Complex Reactions

For any Brønsted-Lowry reaction, use this method:

Find the species that lost a proton - it became a conjugate base
Find the species that gained a proton - it became a conjugate acid
Match each acid with its conjugate base (they differ by one H⁺)

Salt Hydrolysis - Conjugate Pairs in Action

When a salt dissolves in water, its ions may act as acids or bases through hydrolysis:

Salt Type	Example	Ion that Hydrolyzes	Solution pH
Strong acid + strong base	NaCl	Neither	Neutral (pH 7)
Weak acid + strong base	NaCH₃COO	CH₃COO⁻ (base)	Basic (pH > 7)
Strong acid + weak base	NH₄Cl	NH₄⁺ (acid)	Acidic (pH < 7)
Weak acid + weak base	NH₄CH₃COO	Both	Compare Ka vs. Kb

For the last case (both ions hydrolyze), compare Ka of the conjugate acid to Kb of the conjugate base. Whichever is larger determines the pH.

What is the conjugate base of H₂PO₄⁻? What is the conjugate acid of H₂PO₄⁻?

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Conjugate base: HPO₄²⁻ (remove one H⁺). Conjugate acid: H₃PO₄ (add one H⁺). H₂PO₄⁻ is amphoteric - it can act as either an acid or a base. This is common for intermediate species of polyprotic acids.

A solution of potassium cyanide (KCN) is dissolved in water. Is the solution acidic, basic, or neutral?

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Basic. K⁺ is a spectator ion (from the strong base KOH). CN⁻ is the conjugate base of the weak acid HCN. CN⁻ hydrolyzes water: CN⁻ + H₂O ⇌ HCN + OH⁻, producing hydroxide ions. A salt from a strong base and weak acid always gives a basic solution.