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Acid-Base Applications: Understanding Buffers and Titrations

Explore the reactions of weak bases with water, buffered solutions, examples of acid/salt and base/salt buffering pairs, effects of buffers in titration, Henderson-Hasselbalch Equation, and different types of acid-base titrations.

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Acid-Base Applications: Understanding Buffers and Titrations

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  1. Acid & Base Applications Chapter 15

  2. Review:Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.38 x 10-4

  3. Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H2O  BH+ + OH-

  4. Buffered Solutions • A solution that resists a change in pH when either hydroxide ions or protons are added. • Buffered solutions contain either: • A weak acid and its salt • A weak base and its salt

  5. Examples of some Acid/Salt Buffering Pairs The salt will contain the anion of the acid, and the cation of a strong base (NaOH, KOH)

  6. Base/Salt Buffering Pairs The salt will contain the cation of the base, and the anion of a strong acid (HCl, HNO3)

  7. What does a buffer do?And How? • A buffer re….

  8. Titration of an Unbuffered and buffered Solution A solution that is 0.10 M CH3COOH and 0.10 M NaCH3COO is titrated with 0.10 M NaOH A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

  9. Buffer Example Problem • A buffered solution contains 0.5 M HC2H3O2 (Ka=1.8x10-5) and 0.5M NaC2H3O2. • Find pH of the solution • major species: HC2H3O2, Na+, C2H3O2-, H2O

  10. Buffer Example (cont.) • What would happen to the pH if we added 0.010 mol of solid NaOH was added to 1.0 L of this buffered solution?

  11. Buffer Example (cont.) • New major species: HC2H3O2, Na+, OH-, C2H3O2-, H2O • HC2H3O2 + OH- C2H3O2- + H2O • goes to completion because of Strong Base

  12. Buffer Example (cont.) • Now that we know the amount of acetic acid left, we must find out how much H+ is created from it.

  13. Henderson-Hasselbalch Equation

  14. Example of Henderson-Hasselbalch Equation • Find the pH of 0.75 M HC3H5O3 (Ka=1.4x10-4) and 0.25 M NaC3H5O3 • major species: HC3H5O3, Na+, C3H5O3-, H2O

  15. Titrations Made Easy

  16. Weak Acid/Strong Base Titration A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH Endpoint is above pH 7

  17. Strong Acid/Strong Base Titration Endpoint is at pH 7 A solution that is 0.10 M HCl is titrated with 0.10 M NaOH

  18. Strong Acid/Strong Base Titration A solution that is 0.10 M NaOH is titrated with 0.10 M HCl Endpoint is at pH 7 It is important to recognize that titration curves are not always increasing from left to right.

  19. Strong Acid/Weak Base Titration A solution that is 0.10 M HCl is titrated with 0.10 M NH3 Endpoint is below pH 7

  20. Selection of Indicators

  21. Some Acid-Base Indicators

  22. pH Indicators and theirranges

  23. Only shows where the color change occurs

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