This lecture covers the concept of acid-base equilibria, focusing on the pH of weak acids and bases, the dissociation constants, and the relationship between pH and pKa. It also discusses the pH of diacids, the pOH of strong and weak bases, and the pH of salt solutions resulting from strong/weak acid-base reactions. The lecture concludes with examples of pH calculations for various solutions, emphasizing the importance of understanding acid-base equilibria in chemistry and biology.