Section 15.1 Solutions of Acids or Bases Containing a Common Ion Common ion the presence of an ion which appears in both the acid (or base) and a salt in solution. Common ion effect the shift in equilibrium position that occurs because of the addition of an ion already involved in the equilibrium reaction. For example - if a solution of NaF and HF are mixed together, F- would be the common ion. If we look at the dissociation of each compound: NaF(s) H2 O(l ) Na (aq) + F (aq)+ - HF(aq) H+ (aq) + F- (aq) According to Le Chatelier's principle, an increase in concentration of an ion will shift the equilibrium away from the ion added. HF(aq) H+ (aq) + F- (aq)- - Added F ions from NaF. This makes the solution less acidic than a solution of HF alone. Calculate the concentration and the percent dissociation of H+ in 1.0 M HF, and in a mixture of 1.0M HF, and 1.0 M NaF Section 15.2 Buffered Solutions A. Buffered solution A solution that resists a change in pH when either hydroxide ions or protons are added. 1. Buffered solutions contain either: a. A weak acid and its salt b. A weak base and its salt B. Calculations Involving Buffered Solutions Containing Weak Acids 1. "Buffered solutions are simply solutions of weak acids or bases containing a common ion. 2. The pH calculations on buffered solutions require exactly the same procedures introduced in Chapter 14. This is not a new type of problem." 3. "When a strong acid or base is added to a buffered solution, it is best to deal with the stoichiometry of the resulting reaction first. After the stoichiometric calculations are completed, then consider the equilibrium calculations. This procedure can be presented as follows: C. Buffering: How Does it Work? 1. OH- A+ H2O conjugate base + HA Weak acid OH- ions are not allowed to accumulate but are replaced by A- ions 2a. If the amounts of HA and A- originally present are very large compared with the amount of OH- added, the change in [HA]/[A-] will be small. Therefore the pH change will be small 3. Buffering also works for addition of protons instead of hydroxide ions H+ + AHA Conjugate Base Weak acid 1.25X10-3 D. The Henderson-Hasselbalch Equation 1. a. For a particular buffering system (acid-conjugate base pair), all solutions that have the same ratio [A-]/[HA] will have the same pH E. Buffering with a Weak Base and Its Conjugate Acid 1. Weak base B reacts with any H+ added B Base + H+ BH+ Conjugate acid 2. Conjugate acid BH+ reacts with any added OH BH+ + OHB + H2O. Conjugate Acid Base F. Summary 1. Buffered solutions contain relatively large concentrations of a weak acid and the corresponding weak base. They can involve a weak acid HA and the conjugate base A- or a weak base and the conjugate acid BH+ 2. When H+ is added to a buffered solution, it reacts essentially to completion with the weak base present 3. When OH- is added to a buffered solution, it reacts essentially to completion with the weak acid present 4. The pH in the buffered solution is determined by the ratio of the concentrations of the weak acid and the weak base. The pH remains relatively unchanged as long as the concentrations of buffering materials are large compared with the amounts of H+ or OH- added A buffered solution contains 0.5 M acetic acid (HC2H3O2, Ka=1.8 x 10-5) and 0.5 M sodium acetate (NaC2H3O2). Calculate the pH of this solution. Then calculate the change in pH occurs when 0.01 mole of solid NaOH is added to 1.0 L of the buffered solution. Compare the this pH change with that which occur when 0.01 mole of solid NaOH is added to 1.0 L A buffered solution contains 0.25 M NH3 (Kb=1.8 x 10-5) and 0.4 M NH4Cl, calculate the pH of this solution. Then calculate the pH of the solution that results when 0.1 mole of a gaseous HCL is added to 1.0 L of the buffered solution.
