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zumdahl_chemprin_6e_csm_ch08

# zumdahl_chemprin_6e_csm_ch08 - CHAPTER 8 APPLICATIONS OF...

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254 CHAPTER 8 APPLICATIONS OF AQUEOUS EQUILIBRIA Buffers 15. A buffer solution is one that resists a change in its pH when either hydroxide ions or protons (H + ) are added. Any solution that contains a weak acid and its conjugate base or a weak base and its conjugate acid is classified as a buffer. The pH of a buffer depends on the [base]/[acid] ratio. When H + is added to a buffer, the weak base component of the buffer reacts with the H + and forms the acid component of the buffer. Even though the concentrations of the acid and base components of the buffer change some, the ratio of [base]/[acid] does not change that much. This translates into a pH that doesn’t change much. When OH is added to a buffer, the weak acid component is converted into the base com- ponent of the buffer. Again, the [base]/[acid] ratio does not change a lot (unless a large quantity of OH is added), so the pH does not change much. H + (aq) + CO 3 2 (aq) HCO 3 (aq); OH (aq) + HCO 3 (aq) H 2 O(l) + CO 3 2 (aq) 16. When [HA] = [A ] (or [BH + ] = [B]) for a buffer, the pH of the solution is equal to the pK a value for the acid component of the buffer (pH = pK a because [H + ] = K a ). A best buffer has equal concentrations of the acid and base components so it is equally efficient at absorbing H + and OH . For a pH = 4.00 buffer, we would choose the acid component having a K a close to 00 . 4 10 = 1.0 × 4 10 (pH = pK a for a best buffer). For a pH = 10.00 buffer, we would want the acid component of the buffer to have a K a close to 00 . 10 10 = 1.0 × 10 10 . Of course, we can have a buffer solution made from a weak base and its conjugate acid. For a pH = 10.00 buffer, our conjugate acid should have K a 1.0 × 10 10 , which translates into a K b value of the base close to 1.0 × 4 10 (K b = K w /K a for conjugate acid-base pairs). The capacity of a buffer is a measure of how much strong acid or strong base the buffer can neutralize. All the buffers listed have the same pH (= pK a = 4.74) because they all have a 1 : 1 concentration ratio between the weak acid and the conjugate base. The 1.0 M buffer has the greatest capacity; the 0.01 M buffer the least capacity. In general, the larger the concen- trations of weak acid and conjugate base, the greater is the buffer capacity, that is, the more strong acid or strong base that can be neutralized with little pH change. 17. pH = pK a + log ] acid [ ] base [ ; when [acid] > [base], then ] acid [ ] base [ < 1 and log ] acid [ ] base [ < 0. From the Henderson-Hasselbalch equation, if the log term is negative, then pH < pK a . When one has more acid than base in a buffer, the pH will be on the acidic side of the pK a value;

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CHAPTER 8 APPLICATIONS OF AQUEOUS EQUILIBRIA 255 that is, the pH is at a value lower than the pK a value. When one has more base than acid in a buffer ([conjugate base] > [weak acid]), then the log term in the Henderson-Hasselbalch equation is positive, resulting in pH > pK a . When one has more base than acid in a buffer, the pH is on the basic side of the pK a value; that is, the pH is at a value greater than the pK a value. The other scenario you can run across in a buffer is when [acid] = [base]. Here, the log
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