# How do you calculate Keq from pKa values?

Last updated date: 28th Jan 2023
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Hint: The ${K_{eq}}$ in case of the acid dissociation constant is given as ${k_a}$ where ${k_a}$ is the ionization constant for a weak acid. The strength of an acid is defined as the negative logarithm of the acid dissociation constant.

The general equation for the ionization (dissociation into ions) 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 parent acid
${A^ - }$ is the conjugate base
The equilibrium constant for the above reaction is given as shown below.
$k = \dfrac{{[{H_3}{O^ + }][{A^ - }]}}{{[{H_2}O][HA]}}$
The concentration of the water is constant for the reaction taking place in aqueous solution, there the ${H_2}O$ is used as the new quantity known as acid ionization constant which is given as ${K_a}$. The acid ionization constant is also known as acid dissociation constant.
The acid dissociation constant for the reaction is shown below.
${K_a} = K[{H_2}O] = \dfrac{{[{H_3}{O^ + }][{A^ - }]}}{{[HA]}}$
The relation between the ${K_{eq}}$ which in case of acid dissociation constant is used as ${K_a}$ 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
${k_a} = {10^{ - p{k_a}}}$
So by using the above equation, the Keq can be calculated from pKa values.

There is a relation between the $p{k_a}$ and $p{k_b}$of the conjugate acid-base pair.
$p{k_a} + p{k_b} = p{k_w}$
$p{k_w}$ is the constant used for water.
$p{k_a} + p{k_b} = 14$
The value of K and ${k_a}$ differ from each other by the concentration of water. When the value of acid dissociation constant ${k_a}$ is large, stronger is the acid and higher will be the concentration of hydrogen ion ${H^ + }$ at the equilibrium.