# Which of these is the strongest acid? "HCl", "H"_3"PO"_4, "CH"_3"COOH"

Mar 4, 2017

$\text{HCl}$ is the only strong acid in this selection of acids.

The highest $\left[{\text{H}}^{+}\right]$ comes from the acid, $\text{HA}$, that dissociates into ${\text{H}}^{+}$ and ${\text{A}}^{-}$ (its conjugate base) the most:

${\text{HA"(aq) + "H"_2"O"(l) rightleftharpoons "H"_3"O"^(+)(aq) + "A}}^{-} \left(a q\right)$

(note that ${\text{H"_3"O}}^{+}$ and ${\text{H}}^{+}$ are different representations for equivalent species.)

By definition, a strong acid dissociates pretty much completely, so it must give the most ${\text{H}}^{+}$ into solution.

This can be quantified by their acid dissociation constants, ${K}_{a}$:

$\text{HCl}$: ${K}_{a} \approx {10}^{7}$

${\text{H"_3"PO}}_{4}$: ${K}_{a 1} = 6.9 \times {10}^{- 3}$

$\text{CH"_3"COOH}$: ${K}_{a} = 1.76 \times {10}^{- 5}$

Based on the values of the ${K}_{a}$'s, which do you think is the weakest acid (considering only the first proton)?

Mar 4, 2017

Because $H C l$ is the strongest acid...........

#### Explanation:

When we talk of the strength of an acid, we refer to the following equilibrium reaction in water:

$H X \left(a q\right) r i g h t \le f t h a r p \infty n s {H}^{+} + {X}^{-}$

The further the equilibrium lies to the right hand side as we face the page, the STRONGER is the acid. (Sometimes we represent ${H}^{+}$ as the hydronium ion, ${H}_{3} {O}^{+}$, i.e. protonated water:

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

For the LOWER hydrogen halides, $H C l$, $H B r$, $H I$, (but not for HF), the given equilibrium lies almost quantitatively to the right. Thus $1 \cdot m o l \cdot {L}^{-} 1$ solutions of $H C l$, $H B r$, are almost quantitative in ${H}^{+}$, i.e. ${H}^{+} = 1.0 \cdot m o l \cdot {L}^{-} 1$

For acetic acid and phosphoric acid, the equilibrium would not go to completion:

${H}_{3} {\text{CCO"_2H(aq) rightleftharpoons H_3"CCO}}_{2}^{-} + {H}^{+}$

Mind you phosphoric acid has a lower $p {K}_{a 1}$ than acetic acid.