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pH and pOH of Strong Acids and Bases

pH = −log[H⁺] and pOH = −log[OH⁻], related by pH + pOH = 14 at 25 °C. A strong acid or base dissociates completely, so its H⁺ (or OH⁻) concentration equals its concentration, scaled by how many ions each formula unit provides. From that concentration, the pH follows directly through the logarithm.

UNIT 8 TOPIC 8.2 • pH AND pOH OF STRONG ACIDS AND BASES THE pH SCALE Strong acids and bases dissociate completely, so concentration maps directly to pH or pOH. READING THE SCALE · 0 = MOST ACIDIC · 7 = NEUTRAL · 14 = MOST BASIC acidic neutral basic 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.010 M HCl pH = 2 pure water pH = 7 0.0010 M NaOH pH = 11 CORE RELATIONSHIPS pH = −log[H₃O⁺] pOH = −log[OH⁻] pH + pOH = 14 (at 25 °C) Strong acids fully dissociate: 0.010 M HCl → [H₃O⁺] = 0.010 so pH = 2 A LOGARITHMIC SCALE Each 1 pH unit = a 10× change in [H⁺] pH 2 10× more H⁺ pH 3 baseline pH 4 10× less H⁺ pH 2 has 10× more H⁺ than pH 3. STRONG BASE → pH A base gives [OH⁻], so find pOH first, then convert: pOH = −log[OH⁻] pH = 14 − pOH (25 °C) 0.0010 M NaOH → [OH⁻] = 0.0010 pOH = 3 → pH = 11 CED ANCHOR Kw = [H₃O⁺][OH⁻] = 1.0×10⁻¹⁴ at 25 °C — this ties the two scales together: pH + pOH = 14. AP Chemistry · Unit 8 · Acids and Bases
pH = −log[H⁺] and pOH = −log[OH⁻], with pH + pOH = 14 at 25 °C. A strong acid or base dissociates completely, so its ion concentration equals its concentration (times the number of H⁺ or OH⁻ it provides), and the pH follows directly.
pH Meter · Open the sandbox →

The traps misread the pH scale and its bounds, or miscount the dissociation — for example, forgetting that a diprotic strong acid gives two H⁺ per formula unit, or that a lower pH means a higher [H⁺]. Get the ion concentration right, then take the log.

The work

3 ways in · any order
Lesson
pH of Strong Acids/Bases ›

pH and pOH come from log of ion concentrations, and strong acids/bases dissociate completely. The lesson counts ions and computes pH, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 8.2 misconceptions: the pH scale and its bounds misread, and complete strong-acid/base dissociation miscounted.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first