Ka

Meaning: Ka measures how far a weak acid donates its proton to water at equilibrium. Products are the hydronium ion and the conjugate base, so a large Ka means the equilibrium lies toward products and the acid is strong. Water is the solvent and is left out of the expression. Ka changes only with temperature, not with concentration.

Equation: Ka = [H₃O⁺][A⁻] / [HA], where each bracket is a molar concentration at equilibrium (mol/L) and Ka is treated as unitless in most problems. Also pKa = −log Ka. For a conjugate pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C.

On the MCAT: You may be asked to rank acids by strength from Ka or pKa values (largest Ka, smallest pKa is strongest). You may estimate pH of a weak acid solution using [H⁺] ≈ √(Ka × [HA]) when dissociation is small. In titration curves, at the half-equivalence point [HA] = [A⁻], so pH = pKa, which is the center of the buffer region. The Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), builds on Ka. Organic questions also compare acidity by conjugate base stability.

Common confusion: Students think a high Ka means a high pKa. The log has a negative sign, so a lower pKa means a stronger acid. Another trap is mixing up Ka of an acid with Kb of its conjugate base: a strong acid has a very weak conjugate base.