A weak acid is a substance that partially dissociates when it is dissolved in a solvent. In solution there is an equilibrium between the acid, , and the products of dissociation.  
The solvent (e.g. water) is omitted from this expression when its concentration is effectively unchanged by the process of acid dissociation. The strength of a weak acid can be quantified in terms of a dissociation constant, , defined as follows, where  signifies the concentration of a chemical moiety, X.
When a numerical value of  is known it can be used to determine the extent of dissociation in a solution with a given concentration of the acid, , by applying the law of conservation of mass.
where  is the value of the analytical concentration of the acid. When all the quantities in this equation are treated as numbers, ionic charges are not shown and this becomes a quadratic equation in the value of the hydrogen ion concentration value, .
This equation shows that the pH of a solution of a weak acid depends on both its  value and its concentration. Typical examples of weak acids include acetic acid and phosphorous acid. An acid such as oxalic acid () is said to be dibasic because it can lose two protons and react with two molecules of a simple base. Phosphoric acid () is tribasic.
For a more rigorous treatment of acid strength see acid dissociation constant. This includes acids such as the dibasic acid succinic acid, for which the simple method of calculating the pH of a solution, shown above, cannot be used.