Ch16_GilbertChem2E_StudentSolutions - CHAPTER 16 |...

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279 CHAPTER 16 | Equilibrium in the Aqueous Phase 16.1. Collect and Organize In Figure P16.1 four lines are shown to describe the possible dependence of percent ionization of acetic acid with concentration. We are to choose the one that best represents the trend for this weak acid. Analyze The ionization of acetic acid is described by the following chemical reaction: CH 3 COOH( aq ) ± CH 3 COO ( aq ) + H + The degree of ionization is the ratio of the quantity of a substance that is ionized to the concentration of the substance before ionization. Solve According to Figure 16.7, the change in degree of ionization of a weak acid with concentration is not linear and is best described by the red line in Figure P16.1. The degree of ionization increases with decreasing acetic acid concentration. Think About It The percent ionization could be calculated for each concentration if we knew the equilibrium concentration of the acetate ion in solution and the initial concentration of acetic acid dissolved. % ionization = H + equilibrium acetic acid initial 100 16.3. Collect and Organize From Figure P16.3 we are to choose which titration curve represents a strong acid and which represents a weak acid, each of 1 M concentration at the start of the titration. Analyze A strong acid is completely ionized in solution and has a lower initial pH than the weak acid, which is only partially ionized in solution. Solve The blue titration curve represents the titration of a 1 M solution of strong acid. The red titration curve represents the titration of a 1 M solution of weak acid. This is because the pH of the strong acid is expected to be much lower than that of the weak acid at the start of the titration (where no base has yet been added). Think About It Notice that the equivalence point of the titration of the strong acid (pH 7) does not equal that of the weak acid (pH 10).
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280 | Chapter 16 16.5. Collect and Organize For the red titration curve in Figure P16.3 we are to choose the indicator, according to its p K a , that would be best for the titration. Analyze The best indicator is the one with a p K a that is nearest to the end point of the titration. Solve The end point for the red curve is at approximately pH 10. Therefore, the best indicator is the one with a p K a of 9.0. Think About It The lower p K a indicators would show a color change before the end point of the titration was reached. Using these would therefore underestimate the concentration of the weak acid in the original solution. 16.7. Collect and Organize We are shown two titration curves in Figure P16.7. The blue curve has one equivalence point and the red curve has two equivalence points. We are to assign each of these curves to either Na 2 CO 3 or NaHCO 3 . Analyze
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This note was uploaded on 06/12/2011 for the course CHE 132 taught by Professor Hanson during the Spring '08 term at SUNY Stony Brook.

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Ch16_GilbertChem2E_StudentSolutions - CHAPTER 16 |...

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