pKa = −log₁₀(Ka)
pKa = −log₁₀(Ka) provides a convenient logarithmic scale for acid strength. Lower pKa = stronger acid (more dissociation). HCl has pKa ≈ −7 (essentially complete dissociation); acetic acid has pKa = 4.76 (weak acid). The relationship pKa + pKb = 14 (at 25°C) connects an acid's strength to its conjugate base's strength: a strong acid has a weak conjugate base. pKa determines: buffer capacity (Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA])), drug ionization state (affects absorption — unionized forms cross membranes better), amino acid charges at physiological pH, and indicator color change ranges. In organic chemistry, pKa predicts deprotonation sites and reaction mechanisms. The pKa of water itself is ~15.7.