Acids are classified as either strong or weak, based on their ionization in water . A strong acid is an acid which is completely ionized in an aqueous solution. Hydrogen chloride (HCl) ionizes completely into hydrogen ions and chloride ions in water .
A weak acid is an acid that ionizes only slightly in an aqueous solution. Acetic acid (found in vinegar) is a very common weak acid because it doesn’t dissociate very much in solution Its ionization is shown below.
Thus, it is less capable at donating protons.
The ionization of acetic acid is incomplete, and so the equation is shown with a double arrow. The ionization for a general weak acid, HA, can be written as follows:
Because the acid is weak, an equilibrium expression can be written. An acid ionization constant (Ka) is the equilibrium constant for the ionization of an acid.