Chem. 31 – 4/29 Lecture

Announcements Last collected homework (3.1) due today Last quiz today Today’s Lecture –Chapter 9: Polyprotic Acids Dissolution in water Polyprotic acids AS buffers Fraction of acids IN buffered solutions –Chapter 10: Acid-Base Titrations Strong Acid – Strong Base (if time)

Chapter 9 – Polyprotic Acids Dissolution in Water –Acid Forms (e.g M HO 2 CCH 2 CO 2 H – malonic acid) –Intermediate Forms (0.010 M HO 2 CCH 2 CO 2 - ) –Basic Forms (0.010 M PO 4 3- ) –ON LAST EXAMPLE

Chapter 9 – Polyprotic Acids Use as Buffers –Similar to buffers covered in Chapter 8 –With most polyprotic acids where K a1 >> K a2 >> K a3, the Henderson-Hasselbalch equation can be used with a single pK a selected) –The only difficulty can be in deciding which species exist and how to get to those species –Example 1: A mixture of mol NaH 2 PO 4 and mol Na 2 HPO 4 in 1.00 L solution. Determine the pH –Example 2: How many mL of 1.00 M HCl needs to be added to 500 mL of M Na 3 PO 4 to get a pH = 6.5 buffer?

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 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