# What makes hydrochloric acid a strong acid?

Jun 25, 2017

It dissociates in water until nearly 0% of it is left. More specifically, its aqueous acid dissociation constant, ${K}_{a}$, is (much) larger than $1$.

In general chemistry, i.e. in aqueous media, we define hydronium, ${\text{H"_3"O}}^{+}$ to have an acid dissociation constant of $1$, or a $\text{pKa}$ of $0$.

$\text{HCl}$, relative to this reference, has a $\text{pKa}$ of $- 7$, or a ${K}_{a}$ of ${10}^{7}$.

Clearly, ${10}^{7}$ $\text{>>}$ $1$, so $\text{HCl}$ is a very strong acid.

On the other hand, something like acetic acid, ${\text{HC"_2"H"_3"O}}_{2}$, is a weak acid, because it has a ${K}_{a} < 1$, or it has $\text{pKa} > 0$:

${K}_{a} = 1.76 \times {10}^{- 5}$ $\text{<<}$ $1$

$\text{pKa} = - \log \left(1.76 \times {10}^{- 5}\right) \approx 4.75 > 0$

Because this hydronium reference is arbitrary and defined for aqueous media, there are $\text{pKa}$ scales in non-aqueous media.