Unit 8 Notes Topic 13 AP CHEMISTRY The Henderson-Hasselbalch Equation

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📘 Henderson–Hasselbalch Equation

🌟 What’s It For?

This formula helps you calculate the pH of a buffer (a mix of weak acid + salt).

pH = pKa + log ([salt] / [acid])

Use this when you know:

• The pKa (from Ka value)
• The amount of acid and salt in the solution

🧪 Simple Example 1 (Acid Buffer)

What's the pH of a buffer with:

• 0.305 M acetic acid
• 0.520 M sodium acetate
• pKa = 4.76

pH = 4.76 + log(0.520 / 0.305) = 4.99

🧪 Simple Example 2 (Base Buffer)

pOH = pKb + log ([salt] / [base])

0.0400 M ethylamine + 0.0865 M ethyl ammonium chloride, pKb = 3.99

pOH = 3.99 + log(0.0865 / 0.0400) = 4.32 → pH = 14 - 4.32 = 9.68

🔁 What Happens When Acid or Base is Added?

When You Add Acid (H⁺)

• The buffer's salt (CH₃COO⁻) reacts with H⁺
• It becomes more acid (CH₃COOH)
• pH stays nearly the same ✅

When You Add Base (OH⁻)

• OH⁻ grabs H⁺ to form water
• More acid dissociates to give back H⁺
• pH stays nearly the same ✅

📊 What If You Change the Acid or Salt Amounts?

The formula still works! It shows how the pH changes when acid or salt is increased:

• If [acid] = [salt] → pH = pKa
• If [acid] > [salt] → pH is lower than pKa
• If [salt] > [acid] → pH is higher than pKa

pH = pKa + log ([CH₃COO⁻] / [CH₃COOH])

💡 TIP: Buffers work best when acid and salt are close in value. Best range = [acid]:[salt] between 1:10 and 10:1