Nitrous acid (HNO2) is a weak monobasic acid. That means, aqueous nitrous acid (Nitric(III) acid) solution contains very low H+ ion concentration compared to the undissociated HNO2 acid concentration. Though nitric acid is a strong acid, nitrous acid is a weak acid.
In this tutorial, we will discuss following sections.
Nitrous acid is a monobasic weak acid. It means, there is only one hydrogen atom which can release as H+ ion in HNO2 molecule. As that, only limited number of nitrous acid molecules releases H+ ions compared to the existing HNO2 molecules.
Nitrous acid dissociates partially in water to nitrite ion (NO2-) and hydronium ion. This reaction is reversible and generally the equilibrium point is shifted to the left side (In strong acids such as HCl, equilibrium point almost is shifted to right side). That means, concentration of nitrous acid is very much higher than concentrations of nitrite ions and H+ ions.
After writing down the equation of dissociation reaction, expression of dissociation constant is written. Equilibrium concentrations of compounds and ions should be substituted to the terms in the equation. Equilibrium concentrations are determined as following by creating a table.
The equation for dissociation constant can be written as below.
As earlier mentioned, nitrous acid is a weak acid. Therefore, dissociated concentration is very small compared to the initial concentration of nitrous acid.
Assume that we are going to find the pH of 0.1 mol dm-3 nitrous acid solution.
Therefore, we make an assumption of equilibrium concentration of nitrous acid is same as the initial concentration of nitrous acid. Then, we can substitute the equilibrium concentration of nitrous acid (HNO2), nitrite ion (NO2-) and hydronium ion (H3O+) to the equation as following.
In earlier step, we have found that the concentration of H+ (H3O+) ions.
H3O+ concentration given by water is neglected because dissociation of water is much lower compared to the nitrous acid dissociation.
Dissociation constant (Ka) value of nitrous acid at 250C is taken as 5.6 * 10-4 mol dm-3.
|Concentration of nitrous acid (mol dm-3)
When nitrous acid solution is diluted by ten times, it's pH value is increased by 0.5 . As an example, 0.1 mol dm-3 nitrous acid is diluted upto 0.01 mol dm-3, pH value is increased from 2.13 to 2.63 .