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# What is the $p{K_a}$ of acetic acid?

Last updated date: 14th Jul 2024
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Hint :Powerful acids fully dissociate in water, whereas weak acids dissociate just partially. When an acid dissociates, it releases a proton, causing the solution to become acidic, but weak acids have both a dissociated $({A^ - })$ and an undissociated condition $(AH)$ .

$p{K_a}$ (also known as acidity constant or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. In the form of acid–base reactions, it is the equilibrium constant for a chemical reaction called dissociation. $[HA]$ is an acid that dissociates into ${A^ - }$ , the acid's conjugate base, and ${H^ + }$ , a hydrogen ion.
When the concentrations of the system's components do not change over time and all forward and backward reactions occur at the same rate, the system is said to be in equilibrium.
Dissociation equilibrium equation is written as –
$AH \rightleftarrows {A^ - } + {H^ + }$
The acid dissociation constant is the concentration ratio of both sides that is constant under fixed analytical conditions $({K_a})$ . The following equation is used to describe ${K_a}$ .
${K_a} = \dfrac{{[{A^ - }][{H^ + }]}}{{[AH]}}$
The concentration of each variable is indicated by the square brackets. ${K_a}$ expresses how quickly the acid releases a proton using this equation (in other words, its strength as an acid). The equation also demonstrates how the dissociation state of weak acids varies with the concentration of $[{H^ + }]$ in the solution.
Carboxylic acids with $- COOH$ groups, such as acetic and lactic acids, have a ${K_a}$ constant of ${10^{ - 3}}$ to ${10^{ - 6}}$ . As a result, expressing acidity solely in terms of the ${K_a}$ constant can be inconvenient and confusing.
As a result, the $p{K_a}$ index was created to express the acidity of weak acids, and $p{K_a}$ is defined as follows –
$p{K_a} = - {\log _{10}}{K_a}$
The $p{K_a}$ of acetic acid having formula $C{H_3}COOH$ is $4.75$ .

Note :
A stronger acid has a lower $p{K_a}$ value. The lower the value, the more thoroughly the acid dissociates in water. Hydrochloric acid, nitric acid, sulfuric acid, hydrobromic acid, hydroiodic acid, perchloric acid, and chloric acid are the stronger acids.