Question

How do the ${{K}_{a}}$ values for weak and strong acids compare?

Answer 1

The ${{K}_{a}}$ represents the acid dissociation constant which measures the strength of the acid in the solution. The ${{K}_{a}}$ is related with the $p{k_a}$ which is the negative logarithm of acid dissociation constant.

Complete step by step answer:
Dissociation reaction is defined as the reaction where a compound dissociates fully or partially into its constituent ions. It is also known as ionization reaction.
The general equation for the ionization of the weak acids in water is shown below.
$HA(aq) + {H_2}O(l) \to {H_3}{O^ + }(aq) + {A^ - }(aq)$
Where,
HA is the weak acid
${A^ - }$ is the conjugate base
The equilibrium constant for the given reaction is written as shown below.
$k = \dfrac{{[{H_3}{O^ + }][{A^ - }]}}{{[{H_2}O][HA]}}$
The concentration of the water is kept constant for the reaction taking place in aqueous solution, the ${H_2}O$ is now used as the new quantity known as acid ionization constant which is denoted by ${K_a}$. The acid ionization constant is also known as acid dissociation constant.
The acid dissociation constant for the reaction is shown below.
${K_a} = K[{H_2}O] = \dfrac{{[{H_3}{O^ + }][{A^ - }]}}{{[HA]}}$
Thus, an acid dissociation constant ${{K}_{a}}$ is a quantitative measure of the strength of an acid in solution.
The relation between the ${K_a}$ and the $p{k_a}$ is shown below.
The $p{k_a}$ is the negative logarithm of acid dissociation constant which is represented as shown below.
$p{k_a} = - {\log _{10}}{k_a}$
Where,
$p{k_a}$ is the strength of an acid.
So, acid dissociation constant can be written as
${k_a} = {10^{ - p{k_a}}}$
The larger the value of $p{{K}_{a}}$ the smaller will be the extent of dissociation of acid. As weak acid does not completely dissociate into its constituent ions, a weak acid has a $p{{K}_{a}}$ value in the approximate range of -2 to 12 in water. Acids which have the $p{{K}_{a}}$ value less than about -2 are said to be strong acids as a strong acid completely dissociates into its constituent ions.

So, the correct answer is Option A.

Note: Acid dissociation constants are most often related with weak acids, or acids which partially dissociate in solution. This is because strong acids are presumed to dissociate fully in solution and therefore their $p{{K}_{a}}$ values are exceedingly large.