1. Dissociation Equilibria for weak acids and bases
Most acidic substances are weak acids; that is acids that are only partially ionized in aqueous solution (water).  If we represent a general weak acid as HA, we can write the equation for its ionization reaction as follows
Since [H2O] is removed from equilibrium expressions in aqueous solutions the equilibrium constant expression is:

2. Ka is called the acid-dissociation constant
Ka is called the acid-dissociation constant. The magnitude of Ka indicates the tendency of the hydrogen atom to ionize.
Therefore, the ionization reaction for nitrous acid can be written as:-
Where, Ka, is the acid dissociation constant for nitrous acid.
A number of substances behave as weak bases in water. Such substances react with water, removing protons from H2O:
The most commonly weak base is ammonia.
The constant Kb is called the base-dissociation constant.

3. Applications of acid dissociation constants
Ka and Kb are useful for determination of hydronium ion [H3O+] and hydroxide ion [OH-] values.
When the weak acid HA is dissolved in water, two equilibria are established that yield hydronium ions:-

4. [A-] ≈ [H3O+] ……….. (3)
[H3O+] calculated by quadratic equation:

5. Equation (6) can be simplified by making the assumption
CHA – [H3O+] ≈ CHA

6. Ex:
Calculate the hydronium ion concentration [H3O+] in 0.3 M weak acid nitrous acid HNO2?
Ka = 7.1 × 10-4 for HNO2.

8. Ex:
Calculate the hydroxide ion conc. [OH-] and hydronium ion conc. [H3O+] in 9x10-3 M weak base ammonia NH3 .
Kb for NH3 = 1.76 x

12. Buffer solutions
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid that is resistant to a change in pH of a solution when a strong acid or strong base is added.
Buffer solutions are importance in chemistry because they have the property of resisting changes in pH with dilution and with addition of strong acids or bases.

14. Types of buffer solutions
Acidic buffer solutions
An acidic buffer solution has a pH less than 7.
Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt.
A common example would be a mixture of acetic acid (CH3COOH) and sodium acetate (CH3COONa) in solution. If the solution contained equal molar concentrations of both the acid and the salt, it would have a pH of 4.76.
You can change the pH of the buffer solution by changing the ratio of acid to salt, or by choosing a different acid and one of its salts.

15. Alkaline buffer solutions
An alkaline buffer solution has a pH greater than 7.
Alkaline buffer solutions are commonly made from a weak base and one of its salts.
A frequently used example is a mixture of ammonia (NH3) solution and ammonium chloride (NH4Cl) solution. If these were mixed in equal molar proportions, the solution would have a pH of 9.25.
You can change the pH of the buffer solution by changing the ratio of base to salt, or by choosing a different base and one of its salts.

16. Ex: calculate the pH and pOH of a solution that is 0. 28 M NH4Cl and 0
Ex: calculate the pH and pOH of a solution that is 0.28 M NH4Cl and 0.07 M NH3 base.
Kb for NH3 = 1.76 x

19. Ex:
Calculate the change in pH of the buffer solution that formed from 0.4 M formic acid (HCOOH) and 1M sodium formate
( HCOONa) upon dilution by factor of 50. Ka= 1.77x10-4