Weak Acid Dissociation

For a strong acid, like HCl,

HCl dissociates completely to H+ and Cl-

For a strong base, like NaOH,

NaOH dissociates completely to OH- and Na+

For a weak acid,

HA <=> H+ + A- such that the amount of dissociation is a function of the acid dissociation constant. The negative log of this constant is the pKa.

The lower the pKa, the more dissociation and the stronger the weak acid is. The actual amount of dissociation of a weak acid is related to the pH of the solution in which the weak acid exists.

The Henderson Hasselbalch equation relates the pH of the solution to the amount of dissociation. It states that

pH = pKa + log [A-]/[HA]

I often refer to A- as the 'salt' and HA as the 'undissociated acid'

Note that the addition of a strong acid, such as HCl, or a strong base, such as NaOH, to a weak acid solution changes the amount of A- and HA present in it. This is a function of Le Chatelier's principle, which states that a system responds to disruption by trying to reestablish the equilibrium.

For example, if I add HCl (protons) to my weak acid solution, the A- will bind to them, creating HA. This means the reaction is driven to the left.

If I add NaOH, the OH- ions will bind protons, creating water. Thus, protons will be removed and the reaction is driven to the right, because HA will give up protons to replace those lost in the formation of water.