The pH of a solution reflects its concentration of hydrogen ions (H⁺). In everyday terms, a solution with a strong acid—one that readily donates protons—has a lower pH and is considered more acidic than a solution containing a comparable concentration of a weak acid.
For instance, hydrochloric acid (HCl) dissociates completely in water, giving a pH lower than that of a similar concentration of acetic acid (vinegar). Chemically, an undissociated acid is written as HA, and in solution it exists as H⁺ and A⁻ (the conjugate base). Weak acids such as formic acid (HCOOH) partially dissociate, so all three species coexist in varying proportions.
The extent of dissociation, or ionization, depends on the acid’s dissociation constant (Ka). Below we outline how to determine the percentage of ionization using the solution’s pH.
pH is defined as -log₁₀[H⁺], where [H⁺] is the molarity of hydrogen ions in the solution.
Example: For a 0.10 M formic acid solution with pH = 2.5, solve 2.5 = -\log₁₀[H⁺].
Calculation: [H⁺] = 10^(–2.5) = 3.16 × 10⁻³ M (or 3.16 mmol L⁻¹).
For strong acids, you would use the full Ka expression: Ka = ([H⁺][A⁻]) / ([HA] – [H⁺]). However, for weak acids the assumption [H⁺] ≈ [A⁻] holds, and the difference between [HA] and [H⁺] is negligible.
Thus, you can use the initial acid concentration as [HA]. In the example, [HA] = 0.10 M.
The percent ionization is calculated as ([H⁺] / [HA]) × 100.
For our formic acid solution: (3.16 × 10⁻³ M / 0.10 M) × 100 = 3.16 %.
Thus, 3.16 % of the formic acid molecules are ionized at pH 2.5.
