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.