Calculate pH of Nitrous Acid (HNO2) | Examples | Online Calculator

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.

  • Dissociation of nitrous acid acid
  • Calculate pH of nitrous acid by using dissociation constant (Ka) value
  • Online calculator
  • pH values of aqueous nitrous acid solutions in different concentrations

Dissociation of nitrous acid

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.

HNO2(aq) + H2O(l) ⇌ NO2-(aq) + H3O+(aq)

Calculate pH of nitrous acid by using dissociation constant (Ka) value

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.

equilibrium reaction and dissociation of nitrous acid HNO2

Equation for dissociation constant

The equation for dissociation constant can be written as below.

equation for dissociation constant of nitrous acid HNO2

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.

  • [NO2-(aq)] <<< [HNO2(aq)] = 0.1 mol dm-3
  • [H3O+(aq)] <<< [HNO2(aq)] = 0.1 mol dm-3

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.

how to calculate pH of nitrous acid HNO2

Find pH value of nitrous acid

In earlier step, we have found that the concentration of H+ (H3O+) ions.

  • x = [H3O+(aq)] = [NO2-(aq)] = 7.48 * 10-3 mol dm-3

  • pH = -log[H3O+(aq)]
  • pH = -log[7.48 * 10-3]
  • pH = 2.13

Assumptions made in calculations

H3O+ concentration given by water is neglected because dissociation of water is much lower compared to the nitrous acid dissociation.

Online pH calculator of nitrous nitrous acid solution

Dissociation constant (Ka) value of nitrous acid at 250C is taken as 5.6 * 10-4 mol dm-3.


Try: weak acid pH online calculator

pH values of nitrous acid at different concentrations

Concentration of nitrous acid (mol dm-3) pH
0.1 2.13
0.05 2.28
0.01 2.63

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 .