what is the effect of dilution on the fraction of the acetic acid ionized

When a weak acid is diluted, the percent ionization of the acid increases/16%3A_AcidBase_Equilibria/16.06%3A_Weak_Acids). This is because adding more water molecules to the solution will disturb the equilibrium, and the equilibrium will shift in such a way as to reduce the number of water molecules. This means that the equilibrium will shift right, which in turn means that more acid molecules will be ionized. However, the pH of the solution will actually increase, despite the fact that more acid molecules have donated their acidic proton. This happens because of the significant increase in volume in comparison to the relatively small increase in percent ionization.