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Chapter 15 Acid–Base Equilibria

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Chapter 15 Table of Contents Copyright © Cengage Learning. All rights reserved 2 15.1Solutions of Acids or Bases Containing a Common Ion 15.2 Buffered Solutions 15.3 Buffering Capacity 15.4Titrations and pH Curves 15.5 Acid–Base Indicators

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Section 15.1 Solutions of Acids or Bases Containing a Common Ion Return to TOC Copyright © Cengage Learning. All rights reserved 3 Common Ion Effect Shift in equilibrium position that occurs because of the addition of an ion already involved in the equilibrium reaction. An application of Le Châtelier’s principle.

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Section 15.1 Solutions of Acids or Bases Containing a Common Ion Return to TOC Copyright © Cengage Learning. All rights reserved 4 Example HCN(aq) + H 2 O(l) H 3 O + (aq) + CN - (aq) Addition of NaCN will shift the equilibrium to the left because of the addition of CN -, which is already involved in the equilibrium reaction. A solution of HCN and NaCN is less acidic than a solution of HCN alone.

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Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 5 Key Points about Buffered Solutions Buffered Solution – resists a change in pH. They are weak acids or bases containing a common ion. After addition of strong acid or base, deal with stoichiometry first, then the equilibrium.

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Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 6 Henderson–Hasselbalch Equation For a particular buffering system (conjugate acid–base pair), all solutions that have the same ratio [A – ] / [HA] will have the same pH.

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Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 7 Exercise What is the pH of a buffer solution that is 0.45 M acetic acid (HC 2 H 3 O 2 ) and 0.85 M sodium acetate (NaC 2 H 3 O 2 )? The K a for acetic acid is 1.8 × 10 –5. pH = 5.02

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Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 8

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Section 15.3 The MoleBuffering Capacity Return to TOC Copyright © Cengage Learning. All rights reserved 9 The amount of protons or hydroxide ions the buffer can absorb without a significant change in pH. Determined by the magnitudes of [HA] and [A – ]. A buffer with large capacity contains large concentrations of the buffering components.

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Section 15.3 The MoleBuffering Capacity Return to TOC The amount of H + or OH - that buffered solution can absorb without a significant change in pH Buffer capacity measures how well a solution resists changes in pH when acid or base is added. The greater the buffer capacity, the less the pH changes.

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Section 15.3 The MoleBuffering Capacity Return to TOC 2) Magnitudes of [HA] and [A - ] the capacity of a buffered soln. Ex : soln A : 5.00 M HOAc + 5.00 M NaOAc soln B : 0.05 M HOAc + 0.05 M NaOAc pH change when 0.01 mol of HCl(g) is added

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Section 15.3 The MoleBuffering Capacity Return to TOC 3) [A - ] / [HA] ratio the pH of a buffered soln.

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 13 The pH Curve for the Titration of 50.0 mL of 0.200 M HNO 3 with 0.100 M NaOH

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 14 The pH Curve for the Titration of 100.0 mL of 0.50 M NaOH with 1.0 M HCI

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 15 Concept Check Consider a solution made by mixing 0.10 mol of HCN (K a = 6.2 x 10 –10 ) with 0.040 mol NaOH in 1.0 L of aqueous solution. What are the major species immediately upon mixing (that is, before a reaction)? HCN, Na +, OH –, H 2 O

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 16 Let’s Think About It… K = 1.8 x 10 9 HC 2 H 3 O 2 (aq) +OH – C 2 H 3 O 2 – (aq)+ H 2 O Before0.20 mol 0.030 mol0 Change–0.030 mol +0.030 mol After0.17 mol 0 0.030 mol

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 17 Steps Toward Solving for pH K a = 1.8 x 10 –5 pH = 3.99 HC 2 H 3 O 2 (aq) +H2OH2OH3O+H3O+ + C 2 H 3 O 2 - (aq) Initial 0.170 M~00.030 M Change –x+x Equilibrium 0.170 – xx0.030 + x

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 18 Exercise Calculate the pH of a 100.0 mL solution of 0.100 M acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5. pH = 2.87

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 19 Concept Check Calculate the pH of a solution made by mixing 100.0 mL of a 0.100 M solution of acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5, and 50.0 mL of a 0.10 M NaOH solution. pH = 4.74

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 20 Concept Check Calculate the pH of a solution at the equivalence point when 100.0 mL of a 0.100 M solution of acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5, is titrated with a 0.10 M NaOH solution. pH = 8.72

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 21 The pH Curve for the Titration of 50.0 mL of 0.100 M HC 2 H 3 O 2 with 0.100 M NaOH

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 22 The pH Curves for the Titrations of 50.0-mL Samples of 0.10 M Acids with Various K a Values with 0.10 M NaOH

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Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 23 The pH Curve for the Titration of 100.0mL of 0.050 M NH 3 with 0.10 M HCl

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Section 15.4 Titrations and pH Curves Return to TOC P.228 Kjeldahl Nitrogen Analysis : developed in 1883, the analysis remains one of the most widely used methods for determining nitrogen in organic substances such as protein, cereal, and flour.

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Section 15.4 Titrations and pH Curves Return to TOC Melamine contaminated milk

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Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 26 Marks the end point of a titration by changing color. The equivalence point is not necessarily the same as the end point (but they are ideally as close as possible).

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Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 27 The Acid and Base Forms of the Indicator Phenolphthalein

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Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 28 The Methyl Orange Indicator is Yellow in Basic Solution and Red in Acidic Solution

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Section 15.5 Acid–Base Indicators Return to TOC 1) Usually a weak organic acid or base that has distinctly different colors in its nonionized & ionized forms. HIn (aq) H + (aq) + In - (aq) pK HIn nonionized ionized form form

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Section 15.5 Acid–Base Indicators Return to TOC 1) 2) 3)

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Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 31 Useful pH Ranges for Several Common Indicators

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