# Is HCl a strong acid or strong base? How is the determined?

Feb 25, 2017

$\text{Hydrogen chloride}$ is a $\text{strong acid}$.

#### Explanation:

Acidity (and we usually specify an aqueous solvent) is assessed on the basis of the completeness of the following equilibrium:

$H C l \left(g\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + C {l}^{-}$

Hydrogen chloride is acidic because it increases the concentration of the characteristic cation of the solvent, i.e. ${H}_{3} {O}^{+}$, $\text{the hydronium ion}$.

On the other hand, hydrogen fluoride is a weak acid, because the equivalent equilibrium lies farther to the left:

$H F \left(l\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {F}^{-}$

Fluoride is a stronger base than chloride, and hence hydrogen fluoride is a weak acid.