What is a conjugate acid and base pair?

Here's are two examples of conjugate acid-base pair.

The concept of conjugate acid-base pair is related to Bronsted-Lowry acid-base theory and according to this theory, acid is a proton #(H^+)# donor while base is a proton acceptor .

Let's focus on the first example, #CH_3COOH#. It behaves as an acid because it donates a proton and becomes #CH_3COO^-#. Imagine this:
#CH_3COOH - H^+ -> CH_3COO^-#

The product that is formed (#CH_3COO^-#) as a result of the acid, #CH_3COOH# donating its proton is called a conjugate base.

• acid: #CH_3COOH#
• conjugate base: (#CH_3COO^-#)

You might be wondering, why is #CH_3COO^-# called a conjugate base and not conjugate acid. Let's look at the reverse reaction. Going from #CH_3COO^- -> CH_3COOH# will require #CH_3COO^-# to accept proton, making it's a "base".

For the second example, #H_2O#, it behaves as a base because it accepts proton (from #CH_3COOH#) and becomes #H_3O^+#, which is the conjugate acid.

#H_2O + H^+ -> H_3O^+#

In this case,

• base: #H_2O#
• conjugate acid: #H_3O^+#

In short,

• conjugate base is formed when an acid donates a proton.
• conjugate acid is formed when a base accepts a proton.