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ch302 notes ch8.2

# ch302 notes ch8.2 - CH302 Chapter 8 notes part 2 Common Ion...

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CH302 Chapter 8 notes part 2 Common Ion Effects Preparation of Buffer Solutions Consider making an acidic buffer. There are two ways: A: We mix a weak acid with its salt. B: We mix a weak acid and a strong base. In B, some acid is used up creating the salt; the unreacted acid forms the acid part of the buffer. We use the K a expression or the Henderson-Hasselbalch equation (usually easiest) to determine the composition and thus pH. The same methods can be applied to bases... Consider the sodium acetate-acetic acid buffer: NaCH 3 COO Na + + CH 3 COO - CH 3 COOH H + + CH 3 COO - Feeding in the pK a for acetic acid (Ka =1.8x10 -5 ) we have: [ ] [ ] COOH CH COO CH log 744 . 4 pH 3 - 3 + = We can prepare a range of combinations of concentrations of acetate and acetic acid and so acidic buffers over a range* of different pH's can be achieved. e.g., [CH 3 COO - ] = 0.3M ; [CH 3 COOH] = 0.1M then pH = 4.744 + 0.477 = 5.221 (*there are practical limits.. see later) Buffer Calculation Notes/Hints: Often you are asked to prepare a certain amount of a buffer with a certain pH. (A common need in

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