Question

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

Answer 1

`"HCl"` is the only strong acid in this selection of acids.

The highest `["H"^(+)]` comes from the acid, `"HA"`, that dissociates into `"H"^(+)` and `"A"^(-)` (its conjugate base) the most:

`"HA"(aq) + "H"_2"O"(l) rightleftharpoons "H"_3"O"^(+)(aq) + "A"^(-)(aq)`

(note that `"H"_3"O"^(+)` and `"H"^(+)` are different representations for equivalent species.)

By definition, a strong acid dissociates pretty much completely, so it must give the most `"H"^(+)` into solution.

This can be quantified by their acid dissociation constants, `K_a`:

`"HCl"`: `K_a ~~ 10^7`

`"H"_3"PO"_4`: `K_(a1) = 6.9 xx 10^(-3)`

`"CH"_3"COOH"`: `K_a = 1.76 xx 10^(-5)`

Based on the values of the `K_a`'s, which do you think is the weakest acid (considering only the first proton)?

Answer 2

Because `HCl` is the strongest acid...........

Explanation:

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

`HX(aq) rightleftharpoons 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_3O^+`, i.e. protonated water:

`HX(aq) + H_2O(l) rightleftharpoons H_3O^+ + X^-`)

For the LOWER hydrogen halides, `HCl`, `HBr`, `HI`, (but not for `HF)`, the given equilibrium lies almost quantitatively to the right. Thus `1*mol*L^-1` solutions of `HCl`, `HBr`, are almost quantitative in `H^+`, i.e. `H^(+)=1.0*mol*L^-1`

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

`H_3"CCO"_2H(aq) rightleftharpoons H_3"CCO"_2^(-) +H^+`

Mind you phosphoric acid has a lower `pK_(a1)` than acetic acid.