# 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.

Chemistry (university) · Part 08: Acids, bases and electrochemistry · Card 3 of 9 · Last updated 2026-10-19

## Analogy

A weak acid is a shy person at a party who gives away only a few proton 'handshakes'.

## Example

**0.10 M acetic acid, Ka = 1.8×10⁻⁵**

x² / 0.10 = 1.8×10⁻⁵
x = [H⁺] = 1.3×10⁻³ M
pH = 2.87
1.3% 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.

## Related concepts

- [pH and Kw](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-2)
- [Brønsted-Lowry acids and bases](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-1)

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