Chapter 16 Aqueous Ionic Equilibrium Georgia Gwinnett College Chem 1212K Spring 2011 B Shepler Chapter 16 1 Buffers Buffer A solution that resists changes in pH A solution that contains significant amounts of a WEAK ACID and its CONJUGATE BASE ora 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 2 One Very Important Reason to Understand Buffers Buffers any solutions that maintain an approximately constant pH despite small additions of acids or bases Human blood pH 7 4 is maintained by a combination of CO3 2 PO4 3 and protein buffers which accept H Blood pH 7 0 Blood pH 7 8 3 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 Chapter 16 4 The 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 HA aq H 2O l H3O aq A aq A pH pK a log HA base pH pK a log acid Chapter 16 5 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 6 Calculate 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 calculation Chapter 16 7 Example 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 8 Example Calculate the pH of a 1 0 L solution that is 0 040 M in HNO 2 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 4 9 Buffers 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 Effectiveness of Buffers 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 11 Buffer Range 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 12 Buffer Capacity 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 NaCH3COO pH change 0 002 Solution B 0 05 M HCH3COO and 0 05 MNaCH3COO pH change 0 176 13 Example 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 14 Titrations A controlled addition of measured volumes of a solution of known concentration the Titrant from a buret to a second solution of unknown concentration 15 Titrations Strong Acid Titrated with Strong Base 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 16 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 17 Titration of a Strong Base with a Strong Acid 18 Titration of a WEAK acid by a strong base Four Regions Again 1 Before Any Base is Added pH of a weak acid problem 1 Before the Equivalence point Buffer Problem 1 At the Equivalence Point pH of a weak base problem 2 After the Equivalence Point pH of a Strong Base Problem At Each Point We Have Two Steps 1 A Stoichiometry Problem 2 An Equilibrium Problem 19 Titration 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 1 8 x 10 5 What is pH before we add any base 1 Stoichiometry nothing to do 2 Equilibrium Weak Acid Problem mL of RbOH added 20 Titration of weak acid by Strong Base Before the Equivalence Point Consider the titration of 100 0 mL of 0 200 M acetic acid CH3 COOH a weak acid with 0 125 M RbOH a strong base Ka of acetic acid is 1 8 x 10 5 What is pH after 40 0 mL of RbOH are added Buffer Problem 1 Acid Base Stoichiometry 2 Equilibrium HH Equation mL of RbOH added 21 Half Equivalence Point Consider the titration of 100 0 mL of 0 200 M acetic acid CH3 COOH a weak acid with 0 125 M RbOH a strong base Ka of acetic acid is 1 8 x 10 5 What is pH after 80 0 mL of RbOH are added At the half equivalence point HA A pH pKa mL of RbOH added 22 Equivalence Point Consider the titration of 100 0 mL of 0 200 M acetic acid CH3 COOH a weak acid with 0 125 M RbOH a strong base Ka of acetic acid is 1 8 x 10 5 What is pH after 160 0 mL of RbOH are added Moles …