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Chap17-part3

# Chap17-part3 - EXAM 3 April 15th Friday 2011(not on Apr...

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EXAM – 3 : April 15 th Friday 2011 (not on Apr 11 th ) Chapters 15, 16 and 17 REVIEW SESSION-11 Saturday, April 9 th 2011 Chapman 211, 10:30 AM

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Buffer Capacity The capacity of a buffered solution is defined as the “amount” of acid or base that the solution can neutralize before the pH begins to “significantly” change. Consider two identical aqueous buffers with different concentrations: 1 M HC 2 H 3 O 2 2 H 3 O 2 0.1 M HC 2 H 3 O 2 2 H 3 O 2 The buffer capacity of the first buffer will be greater than the capacity of the second buffer. Buffer capacity is not at this point defined quantitatively but qualitatively.
Effective pH Range of a Buffer A given buffer is effective only within a certain pH range . This range is centered on the pK a as can be seen from the Henderson- Hasselbach equation. When [A - ] = [HA], log 10 (1) = 0 and pH = pK a . If enough base is added to raise the concentration of [A-] enough so that [A - ]/[HA] = 10, then pH = pK a + log 10 (10) = pK a + 1. If enough acid is added to raise the concentration of [HA] enough so that [HA]/[A - ] = 10, then pH = pK a + log 10 (0.1) = pK a – 1.

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A buffer contains a weak acid HX and its conjugate base X - . When a strong acid is added to the solution, the H + is consumed to produce HX. HX increases and X - decreases. When a strong base is added to the solution, the OH - is consumed to produce X - . X - increases and HX decreases. How might we determine the pH?

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The beaker on the left contains a mixture of 0.1 M acetic acid and 0.1 M sodium acetate. The beaker on the right contains 0.1 M acetic acid. Both beakers contain methyl orange. (a) Estimate the two pH values. (b) Which solution is better able to maintain its pH when small amounts of NaOH are added?

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(a) Estimate the two pH values. acetic acid & acetic acid sodium acetate left > 4.5 right < 3 The sodium acetate increases the common ion, decreases the [H + ] concentration, and increases the pH.
(b) Which solution is better able to maintain its pH when small amounts of NaOH are added? acetic acid & acetic acid sodium acetate The mixture on the left. This solution is a buffer.

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Acid-Base Titrations In a titration , 1) A solution containing a base is slowly added to a solution containing an acid; or 2) A solution containing an acid is slowly added to a solution containing a base. A plot of the pH as a function of the added material is called a titration curve .
Apparatus for a Titration Using a pH Meter Titration curves can also be found by using colorimetric acid-base indicators and spectrophotometers.

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Titration of a strong acid with a strong base. Given amounts of 0.1 M NaOH are added to 50 mL of 0.1 M HCl. The equivalence point is the place where equal amounts of acid and base are present (pH 7).
1 2 3 4 1) The initial pH is –log 10 (0.1) = 1. 2) As NaOH is added, the added OH

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Chap17-part3 - EXAM 3 April 15th Friday 2011(not on Apr...

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