A weak acid is an acid that dissociates less than 50% in water. The only acids that are not weak acids are strong acids. Weak acids include, but are not limited to:

Despite the name 'weak acids', the corrosive factor of an acid is not dependant upon how strong it is. Only how much an acid dissociates is dependant upon the strength. Hydrofluoric acid, for example, is a weak acid, but has the unique property of being able to etch glass.
The strength of a weak acid is given by its acid constant, ka. This is an application of the equilibrium law, in which the reactant is the acid and the products are the positive and negative ions formed by its dissociation. An acid constant simply shows how much an acid dissociates in solution.

When an acid (weak or strong) dissociates, the products are a hydroxonium ion, H3O+, and a negative ion consisting of the original acid molecule minus a proton, H+. This negative ion is represented as A-. For example when methanoic acid (formic acid, CHOOH) dissociates, the negative ion formed is CHOO-.

Strictly speaking, the acid constant would be given by the concentration of hydroxonium ions formed, multiplied by the concentration of negative ions formed, divided by the concentration of remaining, undissociated acid molecules. This would be represented by:

[H3O+]eqm [A-]eqm
______________
[acid]eqm
However it is possible to make estimations allowing an equation with more easily measurable values. Almost all the hydroxonium ions in an acid solution will have come from the acid, so it is possible to ignore those that come from the dissociation of water. If we assume that all the H3O+ ions come from the acid, then there will be the same number of them as there are A- ions (the negative ions from the acid). In this case, we can substitute the number at the top of the equation for [H3O+]2.

Since so few acid molecules actually dissociate, we can also say that the amount we had at the beginning is almost the same as the amount after equilibrium has been reached.

Thus it is possible to restate the equation for ka as:

[H3O+]2eqm
_________
[acid]start
This allows a calculation of the strength of an acid.
For further information see pH and equilibrium constant expression.

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