Question

What is the Ka of a strong acid like HCl?

Answer 1

The approximate value of `pKa` is `-7` for `HCl`.

Explanation:

Hydrochloric acid is a strong acid and it is considered to completely dissociate in water under the following equation:

`HCl(aq)+H_2O(l)->H_3O^(+)(aq)+Cl^(-)(aq)`

The equilibrium constant `K_a` is written as:

`K_a=([H_3O^+][Cl^-])/(HCl]`

Since the acid dissociates completely, the concentration of products (`[H_3O^+] and [Cl^-]`) is very large and the concentration of the reactant (`[HCl]`) is very small. Therefore, the value of `K_a` is going to be very large (`~10^(7)`).

Since `pK_a=-logK_a`, therefore, the value of the `pK_a` is going to be a negative number.

The approximate value of `pKa` is `-7` for `HCl`.