# How does the acid dissociation constant relate to the strength of the acid?

Sep 7, 2014

The larger the acid dissociation constant, the stronger the acid.

An acid is a substance that can donate a proton (H⁺) to a base.

HA + H₂O ⇌ H₃O⁺ + OH⁻

If the position of equilibrium lies far to the right, the acid is almost completely dissociated. We say that the acid is strong.

We measure the position of equilibrium by the acid dissociation constant, ${K}_{a}$.

${K}_{a} = \text{[H⁺][A⁻]"/"[HA]}$

For hydrochloric acid,

HCl + H₂O ⇌ H₃O⁺ + Cl⁻; ${K}_{a}$ = 1.3 × 10⁶,

${K}_{a}$ is large, so HCl is a strong acid. Only about 1 molecule in a million remains unionized.

If the position of equilibrium lies to the left, we say that the acid is weak.

For acetic acid,

CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺; ${K}_{a}$ = 1.8 × 10⁻⁵

${K}_{a}$ is small, so acetic acid is a weak acid. Only about 4 out of every 100 molecules are ionized.