Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

Acids and Bases

Organic Chemistry

3. Acids and Bases

Acids and Bases

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5:29 minutes

Problem 49

Textbook Question

Write the equation that shows how a buffer made by dissolving CH₃COOH and CH₃COO⁻Na⁺ in water prevents the pH of a solution from changing appreciably when
a. a small amount of H⁺ is added to the solution.
b. a small amount of HO⁻ is added to the solution.

Verified step by step guidance

Step 1: Understand the concept of a buffer solution. A buffer solution resists changes in pH when small amounts of acid (H⁺) or base (OH⁻) are added. In this case, the buffer is composed of acetic acid (CH₃COOH, a weak acid) and its conjugate base, acetate ion (CH₃COO⁻), provided by CH₃COONa.

Step 2: Write the equilibrium reaction for acetic acid in water: CH₃COOH ⇌ CH₃COO⁻ + H⁺. This equilibrium shows how acetic acid can donate protons (H⁺) and how acetate ion can accept protons.

Step 3: Address part (a): When a small amount of H⁺ is added to the solution, the acetate ion (CH₃COO⁻) reacts with the added H⁺ to form acetic acid (CH₃COOH). The reaction is: CH₃COO⁻ + H⁺ → CH₃COOH. This prevents the pH from decreasing significantly.

Step 4: Address part (b): When a small amount of OH⁻ is added to the solution, the acetic acid (CH₃COOH) reacts with the OH⁻ to form water and acetate ion (CH₃COO⁻). The reaction is: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O. This prevents the pH from increasing significantly.

Step 5: Summarize the buffer action: The buffer works by utilizing the equilibrium between CH₃COOH and CH₃COO⁻. When H⁺ is added, CH₃COO⁻ neutralizes it, and when OH⁻ is added, CH₃COOH neutralizes it. This dynamic equilibrium maintains the pH of the solution within a narrow range.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Buffer Solutions

Buffer solutions are mixtures that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, acetic acid (CH3COOH) and its conjugate base acetate (CH3COO−) form a buffer that stabilizes the pH of the solution.

Weak Acid and Conjugate Base Reaction

When a weak acid like acetic acid is present in a solution, it can donate protons (H+) to neutralize added bases. Conversely, the conjugate base (acetate ion) can accept protons to neutralize added acids. This dynamic equilibrium allows the buffer to maintain a relatively constant pH despite the addition of small amounts of H+ or OH−.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the concentration of the weak acid and its conjugate base. It is expressed as pH = pKa + log([A−]/[HA]), where [A−] is the concentration of the conjugate base and [HA] is the concentration of the weak acid. This equation helps predict how the pH will change with the addition of acids or bases.