Unit 8 Notes Topic 1 AP CHEMISTRY

🧪 AP Chemistry Guide: pH & pOH

📊 What is the pH Scale?

• The pH scale ranges from 0 to 14.
• Acids have pH values below 7, bases have pH values above 7, and neutral substances (like water) have a pH of 7.
• The lower the pH, the more acidic the solution.
• The higher the pH, the more basic (alkaline) it is.

🧫 pH of Acids

• Acidic solutions have more H₃O⁺ than OH⁻ ions.
• This means [H₃O⁺] > 10⁻⁷ M → pH is less than 7.
• Higher [H₃O⁺] = Lower pH 📉

🧪 pH of Bases

• Basic solutions have more OH⁻ than H₃O⁺.
• This means [H₃O⁺] < 10⁻⁷ M → pH is greater than 7.
• Higher [OH⁻] = Higher pH 📈

💧 pH of Pure Water

Water at 298 K has equal [H₃O⁺] and [OH⁻], both = 10⁻⁷ M.

pH = –log₁₀[H₃O⁺] = –log(10⁻⁷) = 7

✅ So, water is neutral at pH = 7!

🔬 pH Formula Explained

pH = –log₁₀[H₃O⁺]

[H₃O⁺] = 10⁻ᵖᴴ

➡️ Each decrease of 1 on the pH scale = 10× more acidic!

📊 [H₃O⁺] and pH Table

• 1.0 M → pH = 0
• 0.1 M → pH = 1
• 0.01 M → pH = 2
• ... and so on
• 10⁻ˣ M → pH = x

🧼 What is pOH?

• pOH measures OH⁻ ion concentration.
• pOH = –log₁₀[OH⁻]
• [OH⁻] = 10⁻ᵖᴼᴴ

If you know pOH, use this:

pH = 14 – pOH

Because at 25°C (298K): pH + pOH = 14 🎯

💡 Real Tips for the AP Exam

• Both H⁺ and H₃O⁺ are accepted — use either!
• Remember, pH < 7 = Acid, pH > 7 = Base, pH = 7 = Neutral.
• The pH scale is logarithmic (base 10).
• Every 1-unit change = 10× difference in acidity or basicity.