What is the pH Scale?
pH is a logarithmic scale (0–14) that measures how acidic or basic a solution is. pH = −log[H⁺], where [H⁺] is the concentration of hydrogen ions. pOH measures hydroxide ions similarly. At 25°C, pH + pOH = 14.
pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Each unit change represents a 10-fold change in H⁺ concentration. pOH is calculated the same way using [OH⁻].
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Step-by-step worked examples
Calculate pH if [H⁺] = 0.01 mol/L.
pH = −log[H⁺] = −log(0.01) = −(−2) = 2. The solution is acidic (pH 2 < 7).
If pH = 5, what is [H⁺]?
pH = −log[H⁺] = 5 [H⁺] = 10^(−5) = 0.00001 mol/L.
For a solution with [H⁺] = 10⁻¹¹ mol/L, find pH and pOH.
pH = −log(10⁻¹¹) = 11 pOH = 14 − 11 = 3 (or pOH = −log(10⁻³) = 3).
Flashcards
Quick quiz
Q1.What is the pH formula?
Q2.If [H⁺] = 10⁻⁴, what is pH?
Q3.A solution has pH 11. Is it acidic or basic?
Q4.If pH = 6, what is pOH?
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Common mistakes
Forgetting the negative sign in pH = −log[H⁺]. — Correct: The negative is crucial; pH = −log[H⁺].
Thinking higher [H⁺] = higher pH. — Correct: Higher [H⁺] = lower pH (logarithmic inverse).
Confusing pH with pOH directly. — Correct: pH + pOH = 14; they are related but different.
Assuming each pH unit = 1 order of magnitude. — Correct: Each pH unit = 10× (one order of magnitude) change in [H⁺].
FAQ
How do you calculate pH from H⁺ concentration?
pH = −log[H⁺]. If [H⁺] = 0.001 mol/L, pH = −log(0.001) = 3.
What is pOH and how does it relate to pH?
pOH = −log[OH⁻]. At 25°C: pH + pOH = 14.
Why is the pH scale logarithmic?
H⁺ concentrations vary from 10⁻¹ to 10⁻¹⁴ mol/L, so a logarithmic scale compresses this range.
What is the pH of pure water at 25°C?
pH 7 — pure water has [H⁺] = [OH⁻] = 10⁻⁷ mol/L.