Class Notes Feb 03

Class Notes Feb 03 - 2/3/2010 Announcements: Wednesday,...

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2/3/2010 1 Announcements: Wednesday, February 3, 2010 Homework WebAssign #5- Due Wednesday, February 10 at 8:00 AM Recitation Week 4- “Acid/Base Properties” Lab 2- Buffers Complete prelab prior to recitation. Suggested Textbook Questions Continue Chapter 19 problems. What is the pH of a buffer that is 0.4 M HPO 4 2- and 0.4 M PO 4 3- ? (H 3 PO 4 : pK a1 = 2.14, pK a2 = 7.20, pK a3 = 12.38) February 3, 2010 - CT#1 A. 2.14 B. 7.20 C. 7.60 E. 12.78 D. 12.38 The acid base pair is HPO 4 2- /PO 4 3 . Since HPO 4 2- is the acid, use its pK a . pK a1 is for H 3 PO 4 pK a2 is for H 2 PO 4 - pK a3 is for HPO 4 2- pH = pK a + log(base/acid) pH = pK a3 + log(PO 4 3- /HPO 4 2- ) pH = 12.38 + log(1) = 12.38 Preparing a Buffer (Indirect Method) Second, you can start with the weak acid and add strong base to convert acid to base. How many mL of 0.20 M NaOH must be added to 100 mL of 0.15 M HC 2 H 3 O 2 to get a pH 4.25 buffer? 100 mL x 0.15 M = 15 mmol HC 2 H 3 O 2 Write out the reaction table: HC 2 H 3 O 2 + OH - C 2 H 3 O 2 - + H 2 O I: C: E: 15 mmol x mmol 0 mmol -x mmol -x mmol +x mmol 15-x mmol 0 mmol x mmol Reacts completely!
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Class Notes Feb 03 - 2/3/2010 Announcements: Wednesday,...

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