8.3.A—Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak…

Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak acid or weak base. Acids and Bases UNIT 8

• Weak acids react with water to produce hydronium ions. However, only a small percentage of molecules of a weak acid will ionize in this way. Thus, the concentration of H + 3O is much less than the initial concentration of the molecular acid, and the vast majority of the acid molecules remain un-ionized.
• A solution of a weak acid involves equilibrium between an un-ionized acid and its conjugate base. The equilibrium constant for this reaction is Ka, often reported as pKa. The pH of a weak acid solution can be determined from the initial acid concentration and the pKa. EQN: K [H O] [A ] [HA] a 3= +− EQN: pKa = −log Ka
• Weak bases react with water to produce hydroxide ions in solution. However, ordinarily just a small percentage of the molecules of a weak base in solution will ionize in this way. Thus, the concentration of OH− in the solution does not equal the initial concentration of the base, and the vast majority of the base molecules remain un-ionized.
• A solution of a weak base involves equilibrium between an un-ionized base and its conjugate acid. The equilibrium constant for this reaction is Kb, often reported as pKb. The pH of a weak base solution can be determined from the initial base concentration and the pKb. EQN: K [OH ][HB ] [B] b = −+ EQN: pKb = −log Kb
• The percent ionization of a weak acid (or base) can be calculated from its pKa (pKb) and the initial concentration of the acid (base). The percent ionization can also be calculated from the initial concentration of the acid (base) and the equilibrium concentration of any of the species in the equilibrium expression.
• For any conjugate acid-base pair, the acid ionization constant and base ionization constant are related by Kw: EQN: Kw = Ka × Kb EQN: pKw = pKa + pKb