# Is H_3O^+ hard water? What occurs in an aqueous solution of acetic acid?

##### 2 Answers
Sep 24, 2015

Here's what's going on.

#### Explanation:

First and foremost, ${\text{H"_3"O}}^{+}$ is not hard water, it is the hydronium ion.

Hard water is simply water that has a high concentration of multivalent cations, which are cations that have charges greater than $\left(\text{1+}\right)$.

When acetic acid, $\text{CH"""_3"COOH}$, is added to water, it will ionize to form the acetate anion, ${\text{CH"_3"COO}}^{-}$.

SImply put, acetic acid is a stronger acid than water, so it will donate its acidic proton to a water molecule, which will act as a base and accept this proton.

The water molecule that accepts this proton will form the ${\text{H"_3"O}}^{+}$ cation.

Keep in mind that the reaction does not go to completion because acetic acid is a weak acid in an of itself, so an equilibrium will be established between the unionized and ionized forms

overbrace("CH"_3"COOH"_text((aq]))^(color(green)("acts as an acid")) + overbrace("H"_2"O"_text((l]))^(color(blue)("acts as a base")) rightleftharpoons "CH"_3"COO"_text((aq])^(-) + "H"_3"O"_text((aq])^(+)

In fact, the equilibrium lies to the left, meaning that more acetic acid molecules will not ionize in aqueous solution.

Sep 24, 2015

Because acetic acid is a weak acid, that will to some extent retain the proton bound to the oxygen.

#### Explanation:

Acetic acid undergoes the following equilibrium in water:

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

This is the normal behaviour for an acid, i.e. for the generalized acid, $H A$,

$H A + {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {A}^{-}$

The equilibrium constant, ${K}_{a}$, for acetic acid is $1.76 \times {10}^{-} 5$. This means that these equilibria will lie strongly to the left. Please note that what I write as ${H}_{3} {O}^{+}$ or ${H}^{+}$ are not "hard water". ${H}_{3} {O}^{+}$, the acidium ion, is our conception of a cluster of water molecules with an extra ${H}^{+}$ attached.