Question

g On the basis of their respective pKa values, determine which is the stronger acid, acetic acid (pKa 5 4.7) or benzoic acid (margin; pKa 5 4.2). By what factor do their acidities differ?

Answer 1

Answer:

Benzoic acid is the stronger acid

Explanation:

Weak acids do not dissociate completely in the solution. They exists in equilibrium with their respective ions in the solution.

The extent of dissociation of the acid furnising hydrogen ions can be determined by using dissociation constant of acid ($K_a$).

Thus for a weak acid, HA

$HA \rightleftharpoons A^- + H^+$

The $K_a$ is:

$K_a= \frac{[A^-][H^+]}{[HA]}$

The more the $K_a$, the more the acid dissociates, the more the stronger is the acid.

Also,

$pK_a$ is defined as the negative logarithm of $K_a$.

So, more the $pK_a$, less is the $K_a$ and vice versa

All can be summed up as:

The less the value of $pK_a$, the more the  $K_a$ is and the more the acid dissociates and the more the stronger is the acid.

Given,

$pK_a$ of acetic acid = 54.7

$pK_a$ of benzoic acid = 54.2

$pK_a$ of benzoic acid < $pK_a$ of acetic acid

So, benzoic acid is the stronger acid.