Chemistry / Unit 10 Chem 1844 worked example, 60 to 90 seconds
Solving for pH of a Weak Acid Solution
The idea
Solving for the pH of a weak acid solution means building an ICE table from its initial concentration, substituting the equilibrium row into the Ka expression, applying the small-x approximation where valid, solving for [H3O+], and converting that result to pH by taking its negative base-ten logarithm.
In practice
For 0.100 M HF, Ka = 6.8 × 10⁻⁴: x²/(0.100 − x) ≈ x²/0.100 = 6.8 × 10⁻⁴, so x ≈ 8.2 × 10⁻³. Checking, 8.2 × 10⁻³ is 8.2% of 0.100, above the five-percent threshold, so the full quadratic is solved instead of the approximation, giving x ≈ 7.9 × 10⁻³ M and a final pH near 2.10 once converted.
Where people slip
Accepting a small-x answer without running the percent check risks reporting a pH built on an invalid simplification of the true equilibrium expression.
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