What is the pH Scale?
The pH scale measures acidity and basicity on a 0–14 range. pH 7 is neutral; below 7 is acidic (more H⁺); above 7 is basic (more OH⁻).
pH = −log[H⁺], where [H⁺] is the hydrogen ion concentration. Each step represents a 10× change in acidity. pH <7 is acidic, pH >7 is basic, pH=7 is neutral.
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Step-by-step worked examples
If [H⁺] = 0.01 mol/L, find pH.
pH = −log(0.01) = −log(10⁻²) = 2
If [H⁺] = 1×10⁻⁸ mol/L, find pH.
pH = −log(1×10⁻⁸) = 8 (basic)
Lemon juice (pH≈2) is how many times more acidic than pure water (pH=7)?
pH difference = 7−2 = 5 Factor = 10⁵ = 100,000× more acidic
Flashcards
Quick quiz
Q1.pH = 3 means…
Q2.Neutral solution pH?
Q3.[H⁺]=10⁻¹⁰ mol/L, pH=?
Q4.pH increases by 2 units. H⁺ changes by?
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Common mistakes
Higher pH = more acidic. — Correct: Higher pH = less acidic (more basic). Lower pH = more acidic.
pH directly equals [H⁺]. — Correct: pH is the logarithm of [H⁺]: pH = −log[H⁺].
All pH changes are the same impact. — Correct: pH is logarithmic; each unit is 10×.
pH scale ends at 10. — Correct: Scale goes 0–14 (0=very acidic, 14=very basic).
FAQ
What is the pH of common substances?
Lemon juice≈2, vinegar≈3, milk≈6.5, blood≈7.4, ammonia solution≈11, bleach≈13.
How do you measure pH?
Using pH paper (indicator), pH probe (meter), or calculating from [H⁺] concentration.
Why is pH logarithmic?
Because H⁺ concentrations span 14 orders of magnitude (0.1–10⁻¹⁴ mol/L).
Can pH be above 14 or below 0?
Theoretically yes, but rarely in aqueous solutions. Concentrated acids/bases approach these extremes.