Question

What is conjugate acid and conjugate base ?

Answer 1

All acids have a conjugate base. All bases have a conjugate acid. Acids "donate" H+ when they react. This is most easily seen when they dissociate in water:

$H_{2}S{O}_4+H_{2}O⟶{HSO}_4^{-}+H_3{O}^{+}$

In this example, sulfuric acid ($H_2{SO}_4$ ) is an acid because it "donates" $H^{+}$ to the water. It becomes the hydrogen sulfite ion (${HSO}_4^{-}$ ) which is the conjugate base of sulfuric acid.

The same idea applies to a base:

$N{H}_3+H_{2}O⟶{NH}_4^{+}+{OH}^{-}$

Ammonia (${NH}_3$ ) is a base because is "accepts $H^{+}$ from water to come its conjugate acid, the ammonium ion (${NH}_4^{+}$ ).

Here's another way to look at it.

A conjugate acid contains one more H atom and one more + charge than the base that formed it. Conjugate acid is formed when an acid donates a proton to a base.

A conjugate base contains one less H atom and one more - charge than the acid that formed it. It is left over substance after acid loses its hydrogen ion.

Let us take the example of bicarbonate ions reacting with water to create carbonic acid and hydronium ions.

HCO₃⁻ + H₂O → H₂CO₃ + OH⁻
base + acid → Conj A + Conj B

We see that HCO₃⁻ becomes H₂CO₃. It has one more H atom and one more + charge (-1 + 1 = 0). So H₂CO₃ is the conjugate acid of HCO₃⁻.

The H₂O becomes OH⁻. It has one less H atom and one more – charge. So OH⁻ is the conjugate base of H₂O.