Weak acids and Ka
A weak acid only partly ionizes: HA ⇌ H⁺ + A⁻, with Ka = [H⁺][A⁻]/[HA]. A smaller Ka means a weaker acid. Use an ICE table, and often x is small enough to ignore beside the starting concentration.
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Analogy
A weak acid is a shy person at a party who gives away only a few proton 'handshakes'.
Example
x² / 0.10 = 1.8×10⁻⁵x = [H⁺] = 1.3×10⁻³ MpH = 2.871.3% ionized
In practice
Vinegar tastes sour but is safe to eat because acetic acid ionizes only about 1 percent in solution.
Practice
Flashcards
- Write the Ka expression for HA.Ka = [H⁺][A⁻]/[HA].
Interview questions
- Calculate the pH of 0.10 M acetic acid (Ka = 1.8×10⁻⁵) and state the assumption you make.
- Set up the ICE table
- x²/0.10 = 1.8×10⁻⁵
- x ≈ 1.3×10⁻³ M, pH ≈ 2.87
- Assume x is small versus 0.10 (about 1.3 percent)
Questions people ask
Is a weak acid the same as a dilute acid?
No. Strength is how much an acid ionizes. Concentration is how much acid is dissolved. You can have a dilute solution of a strong acid or a concentrated solution of a weak acid. Both properties affect pH, so always say which one you mean when you describe an acid solution.
How are Ka and Kb of a conjugate pair related?
For a conjugate acid-base pair, Ka × Kb = Kw = 1.0×10⁻¹⁴ at 25 °C. A strong acid therefore has a very weak conjugate base. To find Kb of acetate, divide Kw by Ka of acetic acid: 1.0×10⁻¹⁴ / 1.8×10⁻⁵ = 5.6×10⁻¹⁰.