Question

How do you calculate the pH of a weak acid?

Answer 1

Hint: The weak acids are defined as those acids which on dissolving in water, partially dissociate to give their constituent ions. The hydrogen ion concentration is difficult to calculate for the weak acid.

Complete step by step answer:
Acids are those species, which on dissolving in water give hydrogen ions. According to Lewis theory, the acids are those species which accept the lone pair of electrons donated by the Lewis base.
Depending on their strength, the acid can be divided into strong acid and weak acids.
The strong acids are those acids which on dissolving in water, fully dissociate to give their constituent ions. The strong acids are also referred to as strong electrolyte. Examples of strong acids are hydrochloric acid HCl, sulphuric acid ${H_2}S{O_4}$.
The weak acids are those acids which on dissolving in water, partially dissociate to give their constituent ions. The weak electrolyte is referred to as weak electrolyte. Examples of weak acids are acetic acid, formic acid.
The dissociation of weak acid is shown below.
$HB \to {H^ + } + {B^ - }$
The pH of an acid is defined as the negative logarithm of hydrogen ion concentration. The pH of the acid is generally less than 7.
The pH equation is shown below.
$pH = - \log [{H^ + }]$
For weak acid as it partially dissociates to give its ion. The acid dissociation constant ${K_a}$ is used to find the hydrogen ion ${H^ + }$ concentration.
The acid dissociation constant ${K_a}$ is the quantitative measure of the strength of an acid in the solution.
Where,
${K_a} = \dfrac{{[{H^ + }][{B^ - }]}}{{[HB]}}$

Note:
Don’t get confused by the pH equation used for strong acid and weak acid as it is the same for both the acids. It is just strong acid completely dissociated to give hydrogen ion, so the value of hydrogen ion can be calculated easily.