8.4 Acid-Base Reactions and Buffers

Explain the relationship among the concentrations of major species in a mixture of weak and strong acids and bases. Acids and Bases UNIT 8

• When a strong acid and a strong base are mixed, they react quantitatively in a reaction represented by the equation: H+(aq) + OH−(aq) → H2O(l). The pH of the resulting solution may be determined from the concentration of excess reagent.
• When a weak acid and a strong base are mixed, they react quantitatively in a reaction represented by the equation: HA(aq) + OH−(aq)  A−(aq) H2O(l). If the weak acid is in excess, then a buffer solution is formed, and the pH can be determined from the Henderson-Hasselbalch ( H−H) equation (see
• When a weak base and a strong acid are mixed, they will react quantitatively in a reaction represented by the equation: B(aq) + H3O+(aq)  HB+(aq) + H2O(l). If the weak base is in excess, then a buffer solution is formed, and the pH can be determined from the H−H equation. If the strong acid is in excess, then the pH can be determined from the moles of excess hydronium ion and the total volume of solution. If they are equimolar, then the (slightly acidic) pH can be determined from the equilibrium represented by the equation: HB+(aq) + H2O(l)  B(aq) + H3O+(aq).
• When a weak acid and a weak base are mixed, they will react to an equilibrium state whose reaction may be represented by the equation: HA(aq) + B(aq)  A−(aq) + HB+(aq).