Chem 1120 1st Edition Lecture 16 Outline of Last Lecture I Common Ion Effect II Buffer Solutions a Calculating pH of buffers III Adding Acid to Water and Buffers Outline of Current Lecture I Buffer Capacity II Acid Base Titrations III Titration Calculations IV Titration Curve Current Lecture I If sufficient acid or base is added to a buffer it will be overwhelmed and the pH will change appreciably Buffer Capacity the amount of acid or base a buffer can neutralize before the pH changes significantly 1 More concentrated buffers have a higher buffer capacity For example Buffer A 0 2 M HF and 0 1 M NaF pH 2 87 Buffer B 0 4 M HF and 0 2 M NaF pH 2 87 Buffer B has twice the buffer capacity of Buffer A because it s twice as concentrated 2 Buffer capacity is highest when the concentrations of weak acid and conjugate base are equal 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 HA A 1 M V 1 L find ratio before and after addition of 0 01 moles of OH A HA 1M 1M 1 00 before 1 02 after A HA 1 01M 0 99M Example HA 0 25 M and A 1 75 M V 1L find ratio before and after addition of 0 01 moles of OH A HA 1 75M 0 25M 7 00 before 7 33 after A HA 1 76M 0 24M Blood is a buffered solution its principal buffer is the bicarbonate carbonic acid buffer HCO3 H2CO3 Exhaling CO2 from the lungs maintains the base acid ratio by means of the equilibria LeChatlier This buffer has a much higher capacity to neutralize acids than bases II titration the accurate measurement of the volume of solution required to completely react with a sample During a titration titrant is slowly added form a buret to an analyte in a flask until the equivalence point is reached Titrant is usually a standard solution solution of accurately known concentration Analyte is usually a solution of unknown concentration Equivalence point the point at which stoichiometrically equivalent quantities of reactants are brought together Endpoint the point at which indicator changes color very near to equivalence point Equivalence points for acid base titrations are determined by using indicators that change color near the equivalence point or by continuous monitoring of the pH III Review stoichiometric neutralization calculations Example 27 37 mL of 0 1034 M NaOH are required to neutralize 25 mL of H2SO4 What is the molarity of the H2SO4 General strategy for titration calculations divide the curve into 4 regions 1 before titrant is added single point 2 region up to the equivalence point 3 at the equivalence point single point 4 region after the equivalence point Region 1 Treat as strong acid problem Region 2 Treat as limiting reagent problem Region 3 Treat as complete neutralization Region 4 Treat as limiting reagent problem IV A titration curve is a graph where the y axis is for the analyte pH and the x axis is for the titrant volume added These curves are obtained by adding small portions of titrant to an analyte and after each addition measuring the pH Then you have to plot the pH vs the total volume of titrant added Titra tion curves yield equivalence points the number of replaceable H ions Ka and Kb values and information required for choosing an indicator Variables determining the features of a titration curve acid concentration acid strength Ka number of protons base concentration base strength Kb number of hydroxides analyte acid or base volume of analyte