Final answer:
The 0.0900 M HC2H3O2 solution has the greatest percent ionization due to its lower concentration, and the 4.50 M HC2H3O2 solution has the lowest (most acidic) pH because of its higher concentration of acid providing more hydronium ions.
Step-by-step explanation:
To determine which weak acid solution has the greatest percent ionization and the lowest pH, we look at the initial concentration of the acid and how it ionizes in water. Acetic acid (HC2H3O2), being a weak acid, doesn't dissociate completely in water. The percent ionization is higher when the concentration of the acid is lower because a higher proportion of the acid molecules ionize. Thus, the 0.0900 M HC2H3O2 solution has the greatest percent ionization.
In regards to pH, a lower pH indicates a more acidic solution. Since the 4.50 M solution of HC2H3O2 has the highest concentration of acid, it will contribute more hydronium ions (H3O+) to the solution than the other solutions of lower concentration, leading to a lower (more acidic) pH.