Chem. 31 – 5/4 Lecture

Announcements Lab Stuff –Draft Formal Lab Report (for peer review) due Mon. + Tues. (today and tomorrow) –Peer Review due Wed. + Thurs. in lab –IC and Formal Lab Reports due 5/11 –Check out on 5/13, 5/14 Today’s Lecture –Chapter 9: Polyprotic Acids Fraction of acids IN buffered solutions –Chapter 10: Acid – Base Titrations Strong Acid – Strong Base Titrations

Chapter 9 – Polyprotic Acids Distribution of forms at fixed pH (e.g. when in a buffer) Let’s start with monoprotic acid HA –HA  H + + A - –Fraction present as HA =  (HA) = [HA]/[HA] total = [HA]/([HA] + [A - ]) –Can calculate ratio ([A - ]/[HA]) using Henderson- Hasselbalch equation, but other method calculating  values is more general –Go over method on board –  (A - ) = K a /([H + ] + K a ) (can do derivation yourself)

Chapter 9 – Polyprotic Acids On to a diprotic acid H 2 A From H 2 A form From HA - form From A 2- form Can you guess  (A 2- )? Notice that denominator is the same

Chapter 9 – Polyprotic Acids Example Problems Calculate the pH of M NaNH 2 CH(CH 2 OH)CO 2 (an amino acid: pK a1 = 2.187, pK a2 = 9.209) What is the concentration of CO 3 2- if 1.0 x moles of CO 2 are dissolved into 1.0 L of a buffer at pH = 8.80? (for H 2 CO 3, pK a1 = 6.35, pK a2 = )

Chapter 10 – Acid Base Titrations Main Chapter Goal: Covering shape of titration curves including pH vs. V calculation Why is titration curve shape important? –Affects accuracy and precision of titration –Affects selection of indicator to use Possible Titrations: –Strong Acid – Strong Base Titrations –Monoprotic Weak Acid – Strong Base Titrations –Monoprotic Weak Base – Strong Acid Titrations –Other Titrations: Polyprotic Acids, mixtures of acids

Chapter 10 – Acid Base Titrations Strong Acid – Strong Base Titrations Strong Acid – Strong Base Titration –How does pH Change as NaOH is added? –3 regions to titrations (different calculations in each region): before equivalence point at equivalence point after equivalence point –Go to Board to show pH at 5 mL, 12.5 mL, and 15 mL M NaOH M HCl, 25 mL

Chapter 10 – Acid Base Titrations Strong Acid – Strong Base Titrations M NaOH M HCl, 25 mL Titration Plot

Chapter 10 – Acid Base Titrations Strong Acid – Strong Base Titrations What affects shape of curve? –Concentration of acid and base –Curves are normally sharp. Only exception occurs when intial concentrations are very low (e.g. [H + ] o, [OH - ] o < M) –Note: At low concentrations, one also needs to ensure CO 2 doesn’t affect NaOH solutions Note: Base conc. changed by same factor

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? –Depends on method for determining equivalence point –End point = “measured” volume of titrant –Titration error (systematic) = difference between end point and equivalence point –With indicators, systematic errors occur when pH(end point) ≠ pK a (indicator) – This causes the titration error –With indicators, random errors occur from difficulty in determining correct color shade which depends on [In - ]/[HIn] –A factor of 2 uncertainty in [In - ]/[HIn] (e.g. 0.5 to 2) translates to pH meas = pH true + 0.3

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? Example 1 – precise + accurate titration Uncertainty from judging indicator color (e.g. Methyl orange)  V is small pH Vol. Base

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? Example 2 – mainly imprecise – due to unsharp titration (e.g. low concentrations) Uncertainty from judging indicator color (e.g. Methyl orange)  V is large pH Vol. Base

Chapter 10 – Acid Base Titrations General Acid – Base Titrations What affects accuracy and precision? Example 3 – mainly inaccurate titration (due to selection of wrong indicator small unc. in V but significant error pH Vol. Base equiv. pt. pH indicator color change region (pH < pK a )