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# review3s10 - Dr McCord CH302 OVERVIEW for EXAM 3 Spring...

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Textbook Chapters for Exam 3 You still need to know many of the concepts and calculations from Chapter 7 (type 1 problems). Polyprotic acids are still fair game here and could be part of a buffer. And fraction of species is very useful for buffer systems too. All of Chapter 8 (although you can skip section 3 - I did not cover that) is valid for exam 3. You’re responsible for any material covered in class whether it was on the homework or not. This review sheet covers most everything we have done. Also remember that no equations are given on the exams. Values for equilibrium constants are given where needed. Chapter 7 – Type 1 problems You STILL have to be able to solve type 1 problems (like on exam 2). Type 1 problems are the starting point and the equivalence point on pH curves (titrations). Chapter 8 – Buffers, pH curves, Titra- tions, and Indicators • Know the basic premise for making a buffer solution . These are “type 2” problems according to Dr. McCord (see acid/base help sheets on our web site) • Know HOW a good buffer will neutralize both acid and/or base. Yes, this means know the actual reactions that do the neu- tralizing. • What is buffer capacity (section 8.4)? remember, the amounts (concentrations) determine buffer capacity and the ratio of con- jugates determines the actually pH of the buffer. • Be able to use and identify the Henderson-Hasselbalch equations for both acids and bases. There are help sheets for both of these cases available on our web site. • Be able to calculate the pH of any buffer, acidic or basic. • What is the common ion effect ? Where an ion has more than one source. A - comes a little bit from HA and a lot from NaA, so A - is a common ion. Know the 2 ways to prepare buffers: 1. Mix the two conjugates with proper molar ratios: e.g. HA and NaA 2. Partial neutralization. : e.g. Neutralize a portion of HA with NaOH to MAKE the A - needed. You are essentially doing a titration but stopping somewhere in the middle. After all, that IS where you will have a nice mixture of BOTH acid and conjugate base. Fraction of Species Fraction of species still comes in very handy when it comes to understanding buffers. The buffer region is always centered about the pH that equals p K a for the species. The useful region surrounding that pH is ± 1 pH unit. This is shown on the fraction of species diagram shown below which happen to be for a HA/A - buffer with p K a = 5.00. 1.0 0.8 0.6 f 0.4 0.2 0.0 1 3 5 7 9 11 pH fraction of HA fraction of A - Be able to calculate the new pH of a buffer AFTER the addi- tion of strong acid or strong base. Remember that you will always be subtracting from one species and adding to the other in this calculation. For example, if my acid/conjugate

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review3s10 - Dr McCord CH302 OVERVIEW for EXAM 3 Spring...

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