Unit 8 Notes Topic 14 AP CHEMISTRY Buffer Capacity

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📘 Buffer Capacity – How to Always Get 100% Right Answers

🔍 What is Buffer Capacity?

Buffer capacity tells you how much acid or base you can add to a buffer without causing a big change in pH.

• ⚖️ A buffer with high capacity can handle more acid/base with little pH change.
• 📉 A buffer with low capacity changes pH more easily.

🧪 What Increases Buffer Capacity?

Buffer capacity depends on the concentrations of the weak acid and its conjugate base:

• 🔺 More acid and salt = More capacity
• 📉 Less acid and salt = Less capacity

📈 Buffer Reaction Examples

Let’s say you have a buffer with ethanoic acid (CH₃COOH) and its salt (CH₃COO⁻):

CH₃COOH ⇌ H⁺ + CH₃COO⁻

When Acid (H⁺) is Added:

• CH₃COO⁻ reacts with H⁺ → Forms more CH₃COOH
• pH stays stable because acid is “mopped up”

CH₃COO⁻ + H⁺ ⇌ CH₃COOH

When Base (OH⁻) is Added:

• OH⁻ reacts with H⁺ → Makes water
• CH₃COOH makes more H⁺ to replace it
• pH stays stable again

CH₃COOH + OH⁻ ⇌ CH₃COO⁻ + H₂O

🧠 Worked Comparison

Buffer 1 (High Concentration)

• [CH₃COOH] = 0.80 M
• [CH₃COO⁻] = 0.40 M
• pH = 4.74 + log(0.40/0.80) = 4.44

After 0.05 M NaOH added:

• [CH₃COOH] = 0.75 M
• [CH₃COO⁻] = 0.45 M
• New pH = 4.74 + log(0.45/0.75) = 4.94

Only small pH change → Strong buffer 💪

Buffer 2 (Low Concentration)

• [CH₃COOH] = 0.080 M
• [CH₃COO⁻] = 0.040 M
• pH = 4.74 + log(0.04/0.08) = 4.44

After 0.05 M NaOH added:

• [CH₃COOH] = 0.075 M
• [CH₃COO⁻] = 0.045 M
• New pH = 4.74 + log(0.045/0.075) = 4.93

Still works, but changes pH more → Weaker buffer 🧂

✅ How to Always Get the Right Answer

• 💪 Pick the buffer with higher concentration of both parts
• 🧠 Know: Buffer strength is about amount, not just ratio
• 📊 Use the formula: pH = pKa + log ([salt] / [acid])
• 🔍 Compare concentration values before and after acid/base is added

🧪 Final Practice Question

Q: Which buffer is stronger?

• Buffer A: 50 mL of 0.100 M NaOH + 100 mL of 0.100 M HNO₂
• Buffer B: 0.100 mol NaNO₂ + 100 mL of 1.00 M HNO₂

✅ Answer: Buffer B
Because it has higher concentrations of both acid and salt, meaning it can absorb more H⁺ or OH⁻ without big pH changes!