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CHAPTER 19 IONIC EQUILIBRIA IN AQUEOUS SYSTEMS 19.1 The purpose of an acid-base buffer is to maintain a relatively constant pH in a solution. 19.2 The weak acid component neutralizes added base and the weak base component neutralizes added acid so that the pH of the buffer solution remains relatively constant. The components of a buffer do not neutralize one another when they are a conjugate acid-base pair. 19.3 The presence of an ion in common between two solutes will cause any equilibrium involving either of them to shift in accordance with Le Châtelier’s principle. For example, addition of NaF to a solution of HF will cause the equilibrium H F ( aq ) + H 2 O( l ) ' H 3 O + ( aq ) + F ( aq ) to shift to the left, away from the excess of F the common ion. 19.4 a) Buffer C has equal, high concentrations of both HA and A . It has the highest buffering capacity. b) All of the buffers have the same pH range. The practical buffer range of pH = pKa ± 1, and is independent of concentration. c) Buffer B has the greatest amount of weak base and can therefore neutralize the greatest amount of added acid. 19.5 A buffer is a mixture of a weak acid and its conjugate base (or weak base and its conjugate acid). The pH of a buffer changes only slightly with added H 3 O + because the added H 3 O + reacts with the base of the buffer. The net result is that the concentration of H 3 O + does not change much from the original concentration, keeping the pH nearly constant. 19.6 A buffer with a high capacity has a great resistance to pH change. A high buffer capacity results when the weak acid and weak base are both present at high concentration. Addition of 0.01 mol of HCl to a high-capacity buffer will cause a smaller change in pH than with a low-capacity buffer, since the ratio [HA]/[A ] will change less. 19.7 Only (c) has an affect on the buffer capacity. In theory, any conjugate pair (of any p K a ) can be used to make a high capacity buffer. With proper choice of components, it can be at any pH. The buffer range changes along with the buffer capacity, but does not determine it. A high-capacity buffer will result when comparable quantities (i.e., buffer-component ratio < 10:1) of weak acid and weak base are dissolved so that their concentrations are relatively high. 19.8 The buffer component ratio refers to the ratio of concentrations of the acid and base that make up the buffer. When this ratio is equal to 1, the buffer resists changes in pH with added acid to the same extent that it resists changes in pH with added base. The buffer range extends equally in both the acidic and basic direction. When the ratio shifts with higher [base] than [acid], the buffer is more effective at neutralizing added acid than base so the range extends further in the acidic than basic direction. The opposite is true for a buffer where [acid] > [base].
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This note was uploaded on 02/14/2010 for the course MAE chm2046 taught by Professor Dick during the Spring '10 term at Universidade Federal do Rio de Janeiro.

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