Chem 1120 1st Edition Lecture 15 Outline of Last Lecture I Weak Bases Relation to Weak Acids II Ka and Kb Equilibrium Calculations III Acid Base Properties of Salt Solutions IV Lewis Acids and Bases Outline of Current Lecture I Common Ion Effect II Buffer Solutions a Calculating pH of buffers III Adding Acid to Water and Buffers Current Lecture I A common ion is an ion common to two solutes Common Ion Effect the extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion common with the weak electrolyte OR a shift in the equilibrium caused by the addition of an ion already present in solution These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Example What is the pH of a solution 0 10 M in HCN and 0 20 M NaCN II Buffers solutions that resist pH change when small amounts of acid or base are added What is a buffer comprised of weak acid and its conjugate base OR weak base and its conjugate acid General Case Assume an HX NaX buffer A To calculate the pH of a buffer Consider weak acid HX Take log of both sides Change signs make all terms on both sides negative Substitute pH and pKa Invert log argument This equation above is called the Henderson Hasselbalch Equation Example What is the pH of a buffer prepared from 100 mL each of 0 3 M NH3 and 0 36 M NH4Cl Kb 1 8 X 10 5 Solution Then use the Henderson Hasselbalch equation and plug in the concentrations III Adding acid to water When 5 mL of 0 1 M HCl is added to water what is the pH change Adding acid to buffer When 5 mL of HCl is added to a buffer containing 0 1 M NaA and 0 1 M HA pKa 7 what is the resulting pH To select the acid base pair for a buffer of a specific pH 1 choose an acid whose pKa is equal to the desired pH and 2 solve the Henderson Hasselbalch equation for the base acid ratio Example Which acid and what ratio of base acid concentrations would be best for a pH 5 buffer Buffer range pH range over which the buffer acts effectively Range pH pKa 1 log 10 1 pH pKa 1 log 1 10
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