Chapter 16: Aqueous Ionic EquilibriumBuffersOne Very Important Reason to Understand BuffersCalculate the pH of a Buffer SolutionThe Henderson-Hasselbalch EquationSlide 6Calculate pH Changes in Buffer SolutionsExampleSlide 9Buffers made from a Base plus its Conjugate AcidEffectiveness of BuffersBuffer RangeBuffer CapacitySlide 14TitrationsTitrations: Strong Acid Titrated with Strong BaseExample: Strong Acid Titrated with Strong BaseTitration of a Strong Base with a Strong AcidTitration of a WEAK acid by a strong baseTitration of weak acid by Strong Base: Before we add any baseTitration of weak acid by Strong Base: Before the Equivalence PointHalf-Equivalence PointEquivalence PointAfter the Equivalence PointDifferences Between Strong Acid and Weak Acid Titration CurvesTitration Curves for Different Weak AcidsTitration of a Weak Base with a Strong AcidTitration of Weak Base with Strong AcidTitrations of Polyprotic AcidsChanging Gears (but still doing equilibrium problems)Solubility Product ConstantDifference Between Solubility and Solubility ProductKsp Values For Some Common Ionic SolidsSlide 34Slide 35Slide 36Relative SolubilitiesCommon Ion EffectCommon Ion Effect ExamplePredicting Precipitation ReactionsChapter 16 1Chapter 16: Aqueous Ionic EquilibriumGeorgia Gwinnett CollegeChem 1212KSpring 2011(B. Shepler)Buffers•Buffer: A solution that resists changes in pH–A solution that contains significant amounts of:a WEAK ACID and its CONJUGATE BASE-or-a WEAK BASE and its CONJUGATE ACID•Example: We make a solution by adding NaC2H3O2 to a solution of HC2H3O2–What happens when we add NaOH to this solution?–What happens when we add HCl to this solution?Chapter 16 2One Very Important Reason to Understand Buffers3Buffers any solutions that maintain an approximately constant pH despite small additions of acids or basesHuman blood (pH = 7.4) is maintained by a combination of CO3-2, PO4-3 and protein buffers, which accept H+ Blood pH < 7.0Blood pH > 7.8Calculate the pH of a Buffer Solution•Calculate the pH of a solution that contains 0.100 M NaC2H3O2 and 0.200 M HC2H3O2. (Ka of Acetic acid is 1.8 x 10-5)Chapter 16 4The Henderson-Hasselbalch Equation•When working with buffers we can typically make the assumption that x is going to be small (but you should always check at the end of the calculation).•When working with buffers (AND ONLY WITH BUFFERS) if the x is small approximation is valid we can derive an equation to simplify calculating the pH. (see page 625 for derivation)Chapter 16 5HA(aq)+H2O(l) Û H3O+(aq) +A-(aq)pH =pKa+log[A-][HA]pH =pKa+log[base][acid]Calculate the pH of a Buffer Solution•Calculate the pH of a solution that contains 0.100 M NaC2H3O2 and 0.200 M HC2H3O2. (Ka of Acetic acid is 1.8 x 10-5)•Redo the calculation using the HH equation.Chapter 16 6Calculate pH Changes in Buffer Solutions•Buffers resist changes to pH. However, when an acid or base is added to a buffer the pH does change by a small amount.•Calculating the pH of the resulting solution is a TWO STEP PROCESS.–Step 1: The stoichiometry calculation•Adding Acid converts the conjugate base to the acid•Adding Base converts the acid to the conjugate base–Step 2: The equilibrium calculationChapter 16 7Example•We have 2.0 L of our solution from earlier (0.200 M HC2H3O2 and 0.100 M NaC2H3O2). Calculate the pH after 0.010 mol of solid NaOH are added to the solution.Chapter 16 8Example•Calculate the pH of a 1.0 L solution that is 0.040 M in HNO2 and 0.025 M in Mg(NO2)2. What is the pH after 0.010 mol of HBr is added to the solution? Ka(HNO2) = .4.6×10−49Buffers made from a Base plus its Conjugate Acid•Calculate pH of a solution that contains 0.50 M NH3 and 0.30 M NH4Cl (Kb = 1.8 x 10-5 for NH3).10•A Buffer is composed of a weak acid/base and its conjugate.•Buffers work best when:1. The concentrations of the weak acid/base and salt are similar (within one order of magnitude; buffer range)2. The greater the concentrations of weak acid/base and conjugate, the better (buffer capacity)11Effectiveness of Buffers•We can use the factor of 10 guideline to calculate the buffer range.–Buffers work best when the concentration of acid and base are within a factor of 10 of one another.–Buffers work best in the pH range of pKa ± 1•Buffer capacity is the amount of protons or hydroxide ions that can be absorbed without a significant change in pH.12Buffer Range•Buffers are more effective at higher concentrations of weak acid/base and conjugate partner.–Example: A change in pH occurs when 0.010 mol of HCl (g) is added to 1.0 L of each of the following.(pKa of HCH3COO = 1.8 x 10-5)Solution A: 5.00 M HCH3COO and 5.00 M NaCH3COOpH change = 0.002Solution B: 0.05 M HCH3COO and 0.05 MNaCH3COOpH change = 0.17613Buffer CapacityExample•Which of the following buffer systems would be the best choice to create a buffer with pH = 7.20? Calculate the ratio of concentrations of the buffer components required to make the buffer.HC2H3O2/NaC2H3O2 (Ka=1.8x10-5)NH3/NH4Cl (Kb=1.8x10-5)HClO/KClO (Ka=2.9x10-8)Chapter 16 14Titrations15A controlled addition of measured volumes of a solution of known concentration (the Titrant) from a buret to a second solution of unknown concentration.Titrations: Strong Acid Titrated with Strong Base16•In any titration, we’re particularly interested in four “regions.”•Before titrant is added.•Before the equivalence point.•At the equivalence point.•After the equivalence point.Example: Strong Acid Titrated with Strong Base•Consider the titration of 100.0 mL of 0.100 M perchloric acid with 0.080 M potassium hydroxide. Calculate the pH of the system after each of these volumes of potassium hydroxide have been added: 0.0 mL, 50.0 mL, 125.0 mL, 150.0 mL.17Titration of a Strong Base with a Strong Acid18Titration of a WEAK acid by a strong base•Four Regions Again:1. Before Any Base is Added1. Before the Equivalence point1. At the Equivalence Point2. After the Equivalence Point•At Each Point We Have Two Steps:1. A Stoichiometry Problem2. An Equilibrium Problem19pH of a weak acid problempH of a weak acid problemBuffer ProblemBuffer ProblempH of a weak base problempH of a weak base problempH of a Strong Base ProblempH of a Strong Base ProblemTitration of weak acid by Strong Base: Before we add any base•Consider the titration of 100.0 mL of 0.200 M acetic acid, CH3 COOH, (a weak acid) with 0.150 M RbOH (a strong base). (Ka of acetic acid is