8.2 pH and pOH of Strong Acids and Bases

Strong acids and bases dissociate completely

Strong acids (e.g., HCl, HNO₃, HBr, HI, H₂SO₄, HClO₄) and strong bases (e.g., NaOH, KOH, Ca(OH)₂) ionize 100% in water. This means [H⁺] equals the initial acid concentration, and [OH⁻] equals the initial base concentration (accounting for stoichiometry, like the 2 OH⁻ from Ca(OH)₂).

Calculating pH and pOH

pH = -log[H⁺] and pOH = -log[OH⁻]. At 25°C, pH + pOH = 14, since Kw = [H⁺][OH⁻] = 1.0×10⁻¹⁴. If given [OH⁻] from a strong base, find pOH first, then subtract from 14 to get pH (or vice versa).

Watch the stoichiometry

For strong bases with more than one hydroxide, like Ca(OH)₂, each mole produces 2 moles of OH⁻. So a 0.010 M Ca(OH)₂ solution gives [OH⁻] = 0.020 M, not 0.010 M. Always multiply by the coefficient before taking the log.

Key terms

Strong acid

An acid that ionizes completely in water, so [H⁺] = initial acid concentration (e.g., HCl, HNO₃, H₂SO₄).

Strong base

A base that dissociates completely in water, so [OH⁻] = initial base concentration × number of OH⁻ per formula unit (e.g., NaOH, Ca(OH)₂).

pH formula

pH = -log[H⁺]

pOH formula

pOH = -log[OH⁻]

Relationship between pH and pOH at 25°C

pH + pOH = 14, since Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴

Try an AP-style question

What is the pH of 0.010 M HCl?

1. 2
2. 1
3. 0.01
4. 12