What is the pH Scale?
The pH scale measures how acidic or basic a solution is. pH ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral. It's a logarithmic scale: each step represents a 10-fold change in H⁺ concentration.
pH = −log₁₀[H⁺], where [H⁺] is the molar concentration of hydrogen ions. Similarly, pOH = −log₁₀[OH⁻]. In aqueous solution: pH + pOH = 14 at 25°C.
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Step-by-step worked examples
Calculate pH of a solution with [H⁺] = 0.01 M.
pH = −log[H⁺] = −log(0.01) pH = −log(10⁻²) = −(−2) = 2
Calculate [H⁺] from pH = 11.
pH = −log[H⁺] 11 = −log[H⁺] log[H⁺] = −11 [H⁺] = 10⁻¹¹ M = 1 × 10⁻¹¹ M
A solution has pOH = 3. Calculate pH and [H⁺].
pH + pOH = 14 pH = 14 − 3 = 11 pH = −log[H⁺] 11 = −log[H⁺] [H⁺] = 10⁻¹¹ M
Flashcards
Quick quiz
Q1.If [H⁺] = 10⁻⁵ M, what is pH?
Q2.A solution has pH = 3. Is it acidic or basic?
Q3.If pOH = 2, what is pH?
Q4.Which solution is more acidic: pH 2 or pH 4?
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Common mistakes
Thinking pH is linear with [H⁺]. — Correct: pH is logarithmic: a 1-unit change = 10× change in [H⁺].
Confusing pH > 7 as acidic. — Correct: pH > 7 is basic. pH < 7 is acidic.
Ignoring the negative sign in pH = −log[H⁺]. — Correct: The negative sign is essential: pH = −log, not +log.
Assuming pH + pOH ≠ 14 always. — Correct: pH + pOH = 14 at 25°C in aqueous solution (fundamental rule).
FAQ
What is the pH scale used for?
To measure the acidity or alkalinity (basicity) of a solution on a scale of 0–14.
Why is it a logarithmic scale?
Because [H⁺] varies over a huge range (10⁻¹⁴ to 10⁰), so a log scale makes it manageable.
What is pH 7?
Neutral — neither acidic nor basic. Examples: pure water, buffer solutions at neutral point.
How do you convert pH to [H⁺]?
[H⁺] = 10⁻ᵖᴴ. Example: pH 5 → [H⁺] = 10⁻⁵ = 0.00001 M.