# What is the pH of a .001 M solution of HCl?

Jan 8, 2016

$\text{pH} = 3$

#### Explanation:

Hydrochloric acid, $\text{HCl}$, is a strong acid, which means that it dissociates completely in aqueous solution to form hydronium ions, ${\text{H"_3"O}}^{+}$, and chloride anions, ${\text{Cl}}^{-}$, in a $1 : 1$ mole ratio.

This tells you that every mole of hydrochloric acid will dissociate to produce one mole of hydronium ions and one mole of chloride anions in aqueous solution.

The balanced chemical equation looks like this

${\text{HCl"_text((aq]) + "H"_2"O"_text((l]) -> "H"_3"O"_text((aq])^(+) + "Cl}}_{\textrm{\left(a q\right]}}^{-}$

As you know, the pH of a solution is simply a measure of the concentration of hydronium ions.

color(blue)("pH" = - log(["H"_3"O"^(+)]))

So, if you get one mole of hydronium ions for every one mole of hydrochloric acid, you can say that

["H"_3"O"^(+)] = ["HCl"] = "0.001 M"

This means that the pH of the solution will be

$\text{pH} = - \log \left(0.001\right)$

$\text{pH} = \textcolor{g r e e n}{3}$