Unit 8 Notes Topic 4 AP CHEMISTRY

🧪 Weak Acids & Bases: Ionization Made Easy

Unit 8 | Topic 4 – Weak Electrolytes & Equilibrium

🥽 What Is a Weak Acid?

• Weak acids only partially ionize in water 🌊
• Most molecules stay as HA (not all turn into ions)
• Example: CH₃COOH (acetic acid)

HA ⇌ H⁺ + A⁻

Because of partial ionization, the equilibrium stays on the left.

⚖️ Ka: Acid Dissociation Constant

The equilibrium expression for a weak acid is:

Ka = [H⁺][A⁻] / [HA]

• Ka is usually tiny ➡️ we use pKa for easier math

pKa = –log Ka

Ka = 10⁻ᵖᴷᵃ

💡 Big Ka = stronger acid | Big pKa = weaker acid

📊 Worked Example: Find the pH

Problem: What is the pH of a 0.60 M solution of weak acid HX with pKa = 5.0?

Use ICE Table (Initial, Change, Equilibrium):

HX	⇌	H⁺	+ A⁻
0.60		0	0
–x		+x	+x
0.60 – x		x	x

Step 1: Convert pKa to Ka

Ka = 10⁻⁵ = 1.0 × 10⁻⁵

Step 2: Use Ka expression:

Ka = x² / (0.60)

x² = 1.0 × 10⁻⁵ × 0.60 = 6.0 × 10⁻⁶

x = √6.0 × 10⁻⁶ = 0.00245 M

Step 3: x = [H⁺] → Find pH

pH = –log(0.00245) = 2.61

✅ Final Answer: pH = 2.61

🧪 Weak Bases

• Weak bases also partially ionize in water
• Examples: ammonia (NH₃), methylamine
• Less OH⁻ → lower pH than strong base

B + H₂O ⇌ BH⁺ + OH⁻

Kb = [BH⁺][OH⁻] / [B]

pKb = –log Kb

pH = 14 – pOH

📘 Worked Example: Weak Base

Problem: What is the pH of a 0.40 M NH₃ solution with pKb = 4.7?

Step 1: Convert pKb to Kb

Kb = 10⁻⁴.⁷ = 2.0 × 10⁻⁵

Step 2: Use ICE table & solve like before:

x² / 0.40 = 2.0 × 10⁻⁵ → x² = 8.0 × 10⁻⁶ → x = 0.00283

Step 3: x = [OH⁻] → pOH = –log(0.00283) = 2.55

pH = 14 – 2.55 = 11.45

✅ Final Answer: pH = 11.45

📈 % Ionization Formula

% ionization = ([H₃O⁺] / [HA]) × 100

% ionization (base) = ([OH⁻] / [B]) × 100

💯 Worked Example: % Ionization

Q: What is the % ionization of a 0.10 M acetic acid solution if pH = 3.2?

Step 1: Find [H₃O⁺]

[H⁺] = 10⁻³.² = 6.31 × 10⁻⁴

Step 2: Plug into % formula

% = (6.31 × 10⁻⁴ / 0.10) × 100 = 0.63%

✅ Only 0.63% of the acid ionized!

🧠 Easy Tip: Weak acids/bases use ICE tables + small Ka/Kb values. If pKa or pKb is given, convert first and then solve!