Bronsted–Lowry acid–base identity: What does a Bronsted acid become after it donates a proton (H+)?

Introduction / Context:
The Bronsted–Lowry definition classifies acids as proton donors and bases as proton acceptors. Tracking the species before and after proton transfer clarifies buffering, titration, and enzymatic acid–base catalysis.

Given Data / Assumptions:

• Acid HA donates H+ to a base B.
• Reaction medium is water unless otherwise noted.
• Conjugate pairs differ by exactly one proton.

Concept / Approach:
When HA donates H+, it becomes A−, which is called the conjugate base of HA. Simultaneously, the acceptor (for example, H2O) becomes its conjugate acid (H3O+). This reciprocal relationship underlies buffer action: both members of a conjugate pair coexist near pKa.

Step-by-Step Solution:
Write a generic reaction: HA + H2O → A− + H3O+.Identify donor: HA (acid). After donation, HA → A−.Name A− as the conjugate base of HA.

Verification / Alternative check:
Acetic acid/acetic base pair: CH3COOH ⇌ CH3COO− + H+. The acetate ion is the conjugate base.

Why Other Options Are Wrong:

• Conjugate acid / hydronium: These describe the proton acceptor’s product, not the donor’s.
• Free radical / elemental hydrogen: Not produced in standard acid–base transfer.

Common Pitfalls:
Confusing the identities of conjugate pairs; always compare species that differ by one proton.

Final Answer:
Its conjugate base.