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Class Notes Jan 29

Class Notes Jan 29 - Announcements Friday Homework...

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1/29/2010 1 Announcements: Friday, January 29, 2010 Homework WebAssign #4- Due Wednesday, February 3 at 8:00 AM Recitation Week 3- “Complex Ions” Lab 1- Equilibrium Studies Complete prelab prior to recitation. Suggested Textbook Questions Begin Chapter 19 problems. Calculate the pH of a buffer that is 0.20 M in HNO 2 and 0.02 M in NaNO 2 . (pK a HNO 2 = 3.15) REVIEW: January 27, 2010 - CT#3 B. 3.15 C. 4.15 D. 10.85 E. 11.85 A. 2.15 pH = pK a + log([NO 2 - ]/[HNO 2 ]) pH = 3.15 + log(0.02/0.20) pH = 3.15 + log(0.1) pH = 3.15 1 = 2.15 [acid] > [base] pH < pK a pH will be more acidic! Buffer Range [A - ] [HA] pH = pK a + log pH pK a An acid/base pair can buffer over a pH range of pK a ± 1. HOCl/OCl - is good for buffering pH’s between 6.46 – 8.46 HC 2 H 3 O 2 /C 2 H 3 O 2 - is good between 3.74 5.74 In practice, pick an acid/base pair with a pK a close to the desired pH. Buffer Capacity Two components to the buffer capacity: First, the concentration of the buffer. The higher the concentration of the acid/base pair, the higher the buffer capacity.
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