Knowledge database: Chemical equilibrium: Dissociation constant

The dissociation constant is a numerical value that shows us the relation between the concentrations of undissociated and dissociated forms of molecules that are in equilibrium. A higher dissociation constant means that there is a higher number of dissociated molecules.

The dissociation constant is often mentioned in the context of acids and bases, and because of that we recognize two specific types of constants: dissociation constant of acids and dissociation constant of bases.

The formula for calculating the dissociation constant of acids K_a, and the formula for calculating the dissociation constant of bases K_b can be seen above. Undissociated form of a base is represented by MOH, while their dissociated ions are M⁺ and OH^-. Undissociated form of an acid is represented by HA, and their dissociated ions are H⁺ and A^-.

During the process of acid or base ionization, the concentration of water is considered to be constant, and is thereby not taken into account during the calculation.

The strength of an acid or base is proportional to the extent to which it is dissociated. The strength of acids and bases can be measured by a physical quantity known as the pH. More about pH in the next chapter.