By Mike Gamble | Updated March 24, 2022

The pKa of an acid quantifies its tendency to dissociate in aqueous solution. While strong acids ionize completely, the pKa value is especially useful for weak acids, enabling predictions of pH at known concentrations and aiding in acid identification when the compound is unknown.

Step 1 – Determine the Hydrogen‑Ion Concentration

Calculate [H⁺] from the solution’s pH using the relation [H⁺] = 10^–pH. For example, a pH of 2.29 gives [H⁺] = 5.13 × 10⁻³ mol L⁻¹.

Step 2 – Compute the Dissociation Constant (Ka)

Use the expression Ka = [H⁺]² / ( [HA] – [H⁺] ), where [HA] is the initial acid concentration. With an initial concentration of 0.15 mol L⁻¹, Ka = (5.13 × 10⁻³)² / (0.15 – 5.13 × 10⁻³) = 1.82 × 10⁻⁴.

Step 3 – Convert to pKa

Finally, obtain the pKa via pKa = –log₁₀(Ka). Using the Ka above, pKa = –log₁₀(1.82 × 10⁻⁴) = 3.74.