Summary of approach
Weak acids and weak bases react with water to less than 5%.
The initial concentration and the equilibrium concentration of the member of the pair that is dissolved can be considered to be approximately equal.
When a single member of the conjugate pair is dissolved, the stoichiometry of its reaction with water (shown below) can be used to relate the equilibrium concentration of its conjugate to pH.
*Each acid reacts to give one H3O+ and one base:
acid + H2O
H3O+ + conjugate base
**Each base reacts to give one acid and one OH–:
base + H2O conjugate acid + OH–
The initial concentration and the equilibrium concentration of the member of the pair that is dissolved can be considered to be approximately equal.
| If solute is | [A–] equals | [HA] equals |
| only the acid of the pair | [H3O+]* | initial c(acid) |
| only the base of the pair | initial c(base) | Kw/[H3O+]** |
| both the acid and the base of the pair | initial c(base) | initial c(acid) |
When a single member of the conjugate pair is dissolved, the stoichiometry of its reaction with water (shown below) can be used to relate the equilibrium concentration of its conjugate to pH.
*Each acid reacts to give one H3O+ and one base:
acid + H2O
H3O+ + conjugate base**Each base reacts to give one acid and one OH–:
base + H2O
conjugate acid + OH–