Chapter 17 Ionic Equilibria of Weak Electrolytes -weak acids and weak bases are probably the most significant weak electrolytes 17-1 The Ionization of Water -water is an extremely weak electrolyte that undergoes self-ionization, with the formation of very small, but equal amounts of hydronium and hydroxide ions H2O + H2O <--> H3O+ + OH- -this slight ionization plays an important role in acid-base reactions in water -pure water ionizes to provide equal numbers of hydronium and hydroxide ions and is therefore neutral 17-2 pH and pOH -equations are in your notes from section 16-5) ex.1 The concentration of the hydrochloric acid secreted by the stomach after a meal is about 1.2 X 10-3 M. What is the pH of stomach acid? Ex.2 Calculate the pH of a 0.10M solution of nitric acid (a strong acid) and of 0.10M solutions of acetic acid (1.3% ionized) and nitrous acid (6.5% ionized). Ex.3 Water in equilibrium with air contains 4.4 X 10-5% carbon dioxide. The resulting cabonic acid, H2CO3, gives the solution a hydrogen ion concentration of 2.0 X 10-6 M, about 20 times larger than that of pure water. Calculate the pH of the solution Ex. 4. Calculate the hydrogen ion concentration of a solution whose pH is 9.0 Ex. 5 Calculate the hydrogen concentration of blood, the pH of which is 7.3 Ex.6 What are the pOH and pH of a 0.0125 M solution of potassium hydroxide, KOH? 17.3 Ion Concentrations of Solutions of Strong Electrolytes -strong electrolytes are completely ionized in aqueous solution, so the ion concentrations may be found directly from the molar concentration. 17-4 The Ionization of Weak Acids -w can tell by measuring the pH of an aqueous solution of a weak acid such as acetic acid CH3CO2H, that only a fraction of the molecules are ionized into hydrogen cations and acetate anions. -the remaining acid is not ionized Ex. 7 Calculate the percent of the moles of water present that are consumed in the formation of hydronium ions in the ionization reaction for 0.10M acetic acid (1.3% ionized). CH3CO2H (aq) + H2O <--> H3O+ (aq) + CH3CO2- (aq) Ex. 8 In a 0.0800M solution acetic acid is 1.50% ionized. Calculate H+, CH3CO2- , and CH3CO2H in the solution. Ex.9 Using the concentrations from ex 8, calculate the ionization constant of acetic acid. 17.5 The Ionization of Weak Bases. -the most common weak base is aqueous ammonia ex. The sedative Veronal C8H12N2O3 is a weak base with an ionization constant Kb, of 1.1X 10-8. What is the OH- in a 0.010 M aqueous solution of Veronal? 17.6 The Salt Effect -the extent of ionization of a weak electrolyte may be influenced by the presence of other ions, the addition of a salt slightly increases the degree of aqueous solution 17.7 The Common Ion Effect -the acidity of an aqueous solution of acidic acid decreases when a stong electrolyte like sodium acetate is added -this is explained by Le Chatelier's principle (if stress is applied to a system in equilibrium, the equilibrium shifts in a way that tends to undo the effect of the stress. -the addition of acetate ions causes the equilibrium to shift to the left in CH3CO2H <--> H+ + CH3CO2- -the shift in equilibrium caused by the addition of a substance with an ion in common with the substances in equilibrium Ex. Calculate H+ in a 0.10 M solution of CH3CO2H that is 0.50M with respect to NaCH3CO2. Ex. A 0.10 M solution of aqueous ammonia, also containing ammonium chloride has a hydroxide ion concentration of 2.8 X 10-6 M. What is the concentration of ammonium ion in the solution? Ex. Exactly 10 mL of 0.100 M sodium hydroxide solution is added to 25 mL of 0.100 M acetic acid solution. Calculate the hydrogen ion concentration of the resulting 35 mL of solution. 17.8 Buffer Solutions -mixtures of weak acids and their salts or mixtures of weak bases and their salts are called buffer solutions or buffers. -buffers resist a change in pH when small amounts of base or acid are added Ex. Calculate the pH of a buffer that is a mixture containing 0.10M acetic acid and 0.10M sodium acetate. Ex. Calculate the change in pH when 1.0 mL of 0.10 M NaOH is added to 100 mL of the above buffer. Ex. Calculate the change in pH when 1.0 mL of 0.10M HCl is added to 100 mL of the buffer used above. Ex. Calculate the pH of an unbuffered solution containing 1.8 X 10-5 M HCl. -buffer capacity is the amount of an acid or base that can be added to a certain volume of a buffer solution before the pH begins to change significantly. -a good buffer mixture should have about equal concentrations of the weak acid and its salt or of the weak base and its salt -a buffer solution has generally lost its usefulness when one component of the buffer pair is less than about 10% of the other. -weak acids and their salts are better as buffers for pH <7: weak bases and their salts are better as buffers for pH >7 17.9 The Ionization of Weak Diprotic Acids -polyprotic acids, those from which more than one proton may be removed, ionize in water in successive steps. Ex. The concentration of H2S in a saturated aqueous solution of the gas at room temperature is about 0.10M. Calculate H+, HS-, and S2- of the solution. Ex. Calculate the concentration of sulfide ion at equilibrium in a satureated solution of hydrogen sulfide to which HCl has been added to make the hydrogen ion concentration of the solution 0.10 M at equalibruim. (A saturated solution of H2S is 0.10 M in hydrogen sulfide.) 17.10 The Ionization of Weak Triprotic Acids -a triprotic acid ionizes water in 3 steps for phosphoric acid Step 1 H3PO4 <--> H+ + H2PO4- Step 2 H2PO4- <--> H+ + HPO42- Step 3 HPO42- <--> H+ + PO43- Ex. Calculate the concentrations of H3PO4, H2PO4-, HPO42-, PO43-, H+, and OH- present at equilibrium in a solution containing a total phosphoric acid concentration of 0.100 M. 17.11 Omit 17.12 Reaction of a salt with a strong base and a weak acid with water. -a salt formed from a strong base and a weak acid contains the conjugate base of the weak acid. Ex. Calculate the hydroxide ion concentration, the percent reaction, and the pH of a 0.050 M solution of sodium acetate. Ex. Calculate the equilibrium constant for the reaction of the sulfide ion with water and the sulfide ion concentration in a 0.0010M solution of sodium sulfide. 17.13 through 17.17 omit