Unit 8 Notes Topic 12 AP CHEMISTRY Properties of Buffers

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🧪 Topic 12: Properties of Buffers

💡 What is a Buffer?

A buffer solution resists changes in pH when small amounts of acid or base are added. It's like a chemical cushion!

• 🧊 Made of: Weak acid + Conjugate base OR Weak base + Conjugate acid
• 🧪 Helps keep pH nearly constant (but only for small additions)

🔬 Types of Buffers

Acidic Buffer

Example: CH₃COOH (ethanoic acid) + CH₃COONa (sodium ethanoate)

CH₃COOH ⇌ H⁺ + CH₃COO⁻

Basic Buffer

Example: NH₃ (ammonia) + NH₄Cl (ammonium chloride)

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

🌍 Real-Life Buffer Examples

Application	Buffer Pair
Human blood	H₂CO₃ / HCO₃⁻
Shampoo	Citric acid / citrate
Pool water	H₂CO₃ / HCO₃⁻
Fish tank	H₂PO₄⁻ / HPO₄²⁻
Cheese making	Lactic acid / lactate

🧠 20 Types of Buffer Questions

1. What is a buffer?
2. What does a buffer do?
3. Name two examples of buffer pairs.
4. What’s the weak acid in CH₃COOH / CH₃COONa?
5. Can HCl + NaCl form a buffer?
6. Which of these forms a buffer? NH₃ + NH₄Cl or HCl + HNO₃?
7. What happens when acid is added to a buffer?
8. What happens when base is added to a buffer?
9. What is a conjugate base?
10. What is the pH range of effectiveness for a buffer?
11. What’s the purpose of NH₄Cl in a buffer with NH₃?
12. Write the buffer equation for HF and F⁻.
13. Can NaOH + KOH form a buffer? Why or why not?
14. Does a buffer keep pH perfectly constant?
15. What’s the best [HA]/[A⁻] ratio for a buffer?
16. Why can a buffer only resist small pH changes?
17. Can strong acid + strong base ever form a buffer?
18. Why is blood a good buffer system?
<19>Why do buffers matter in biology?
19. What does it mean when pH = pKa in a buffer?

🧪 20 Practice Problems (Beginner to Medium)

1. Which of the following pairs can act as a buffer: A) HCl/NaCl B) CH₃COOH/CH₃COONa C) NaOH/KOH?
2. Identify the conjugate base in NH₄⁺/NH₃ buffer.
3. True or False: NH₃ and NaOH make a buffer.
4. Write the buffer equilibrium for CH₃COOH + CH₃COONa.
5. Which buffer would be best for pH ≈ 4.5: A) NH₃/NH₄Cl B) CH₃COOH/CH₃COONa C) NaOH/NaCl?
6. What happens to a buffer if too much H⁺ is added?
7. Explain why HCl and HNO₃ can’t form a buffer.
8. What’s the pH of a buffer with [HA] = [A⁻] and pKa = 4.76?
9. In NH₃/NH₄⁺, what happens if OH⁻ is added?
10. A buffer has pKa = 5. What pH range will it work best in?
11. Why does CH₃COOH/CH₃COONa buffer resist acid?
12. Which buffer pair is best for pH 9.5?
13. Can H₂SO₄ + NaOH be a buffer? Why not?
14. If pH = pKa, what does that say about the concentrations of acid/base?
15. A solution is made from 0.25 M CH₃COOH and 0.25 M CH₃COONa. What’s the pH? (pKa = 4.76)
16. Can a buffer be made from NH₃ and NaOH?
17. Choose the correct buffer combo: A) NH₃ + HNO₃ B) HCl + NaOH C) CH₃COOH + CH₃COONa
18. What’s wrong with a buffer made from only CH₃COOH and no salt?
19. Why does a buffer fail when strong base is added in large amounts?
20. Which is more effective: buffer with 1:1 ratio or 1:1000 ratio of HA/A⁻?