Why do we assume 100% ionization for acetic acid (a weak acid)? This is a titration problem. The titrant is sodium hydroxide, if that helps.
Acetic acid IS, by all means, a weak acid. It does not ionize completely but only about 1% or so depending upon the concentration. My assumption about your question is that as acid is titrated with NaOH that the acid becomes weaker and weaker. At weaker concentrations it ionizes more than that and at the end has completely ionized.
Try this exercise.
% ionization 0.1M acetic acid. I get 1.3%.
% ionization 0.001 M acetic acid is 13%
I didn't do it for less but you get the picture. When you get to 1 drop before the end point it is essentially 100% ionized.
Thx poutana
Apark22tv
When performing a titration, it is common to assume 100% ionization for strong acids and bases, but not for weak acids and bases. However, acetic acid (CH3COOH) is a special case. Despite being a weak acid, it undergoes almost complete ionization when dissolved in water.
The reason for this is that when acetic acid dissolves in water, it forms an equilibrium between the undissociated acid molecules (CH3COOH) and its ions (CH3COO- and H+). This equilibrium strongly favors the dissociation of acetic acid to form acetate ions and hydrogen ions.
The extent of ionization for a weak acid is typically described by its acid dissociation constant (Ka). Acetic acid has a relatively small Ka value, indicating that it is a weak acid. However, the actual degree of ionization for acetic acid, even though it is a weak acid, is close to 100% due to the presence of water molecules and the specific chemical properties of acetic acid.
Now, when titrating acetic acid with a strong base like sodium hydroxide (NaOH), the sodium hydroxide dissociates completely into sodium ions (Na+) and hydroxide ions (OH-). The hydroxide ions (OH-) rapidly react with the acetic acid (CH3COOH), causing it to undergo ionization and form acetate ions (CH3COO-) and more water molecules.
In summary, even though acetic acid is technically a weak acid, it undergoes almost complete ionization when dissolved in water. This assumption makes the titration calculations more straightforward and allows us to calculate the concentration of acetic acid using stoichiometry and the balanced equation for the reaction between the acid and base.