# Is acetic acid a weak acid or strong acid? Why?

Jun 28, 2017

$\text{Acetic acid is a WEAK acid.........}$

#### Explanation:

The strength of an acid in aqueous solution depends on the extent of the equilibrium, i.e. the protonolysis reaction......

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

For strong acids, this equilibrium lies STRONGLY to the right, and there is little of the parent acid, $H X$, left undissociated......

On the other hand, for weak acids, the equilibrium reaction lies to the left, and at equilibrium there are significant quantities of the parent acid, and reduced concentrations of the hydronium ion......

And we may thus link acid strength with ${K}_{a}$, $\text{the acid dissociation constant}$. If ${K}_{a}$ is large (or conversely, where $p {K}_{a} = - {\log}_{10} {K}_{a}$ is small or negative) we gots a strong acid. The cutoff points are fairly arbitrary. The mineral acids, $H X \left(X \ne F\right) , {H}_{2} S {O}_{4} , H N {O}_{3} , H C l {O}_{4}$, are all strong acids, and at equilibrium they are near stoichiometric in ${H}_{3} {O}^{+}$.

And of course, for weak acids, ${K}_{a}$ is fairly small. For acetic acid, ${K}_{a} = {10}^{- 4.76} = 1.74 \times {10}^{-} 5$, and at equilibrium there is a significant concentration of $H O A c$.........Try solving this for a $1 \cdot m o l \cdot {L}^{-} 1$ solution, and calculate the percentage dissociation.

Jun 28, 2017

It is a weak acid.

#### Explanation:

Acetic acid, $\text{CH"_3"COOH}$, is a weak acid, because it is present in solution primarily as whole $\text{CH"_3"COOH}$ molecules, and very little as ${\text{H}}^{+}$ and ${\text{CH"_3"COO}}^{-}$ ions.

${\text{CH"_3"COOH"(aq) rightleftharpoons "H"^+(aq) + "CH"_3"COO}}^{-} \left(a q\right)$

The acid dissociation constant (${K}_{a}$) of acetic acid is $1.76 \times {10}^{-} 5$. This basically means that the ratio of $\text{CH"_3"COOH}$ concentration to the concentration of ${\text{H}}^{+}$ and ${\text{CH"_3"COO}}^{-}$ ions is $1 : 1.76 \times {10}^{-} 5$:

$\left(\left[\text{H"^+]["CH"_3"COO"^(-)])/(["CH"_3"COOH}\right]\right) = 1.76 \times {10}^{-} 5$

Which furthermore indicates that acetic acid is weak, because strong ions ionize almost completely.