What is a conjugate base?

A conjugate base is the species that is formed when an acid donates a proton (H⁺) during a chemical reaction. When acids react with bases, they tend to release protons. The acid that loses the proton is transformed into its conjugate base. The relation between an acid and its conjugate base can be represented as:

HA (Acid) → H⁺ (Proton) + A⁻ (Conjugate Base)

Here, HA represents the acid, H⁺ is the proton that is released, and A⁻ is the conjugate base. The conjugate base is characterized by one less hydrogen atom and an increased negative charge due to the loss of the positively charged hydrogen ion. This concept is an integral part of the Bronsted-Lowry acid-base theory, which defines acids as proton donors and bases as proton acceptors.

As an example, if we start with hydrochloric acid (HCl), after it donates a proton, the conjugate base that remains is the chloride ion (Cl⁻):

HCl (Acid) → H⁺ (Proton) + Cl⁻ (Conjugate Base)