What is the pH and pOH Formula?
pH and pOH are logarithmic scales that quantify how acidic or basic a solution is. They're related by a simple rule: at 25°C, pH + pOH = 14, because the product of [H⁺] and [OH⁻] is always 10⁻¹⁴.
pH = −log[H⁺] and pOH = −log[OH⁻]. Since [H⁺][OH⁻] = 10⁻¹⁴ at 25°C, adding pH + pOH always gives 14. Low pH = acidic, high pH = basic.
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Step-by-step worked examples
A solution has [H⁺] = 0.001 M. Find pH.
pH = -log[H⁺] = -log(0.001) 0.001 = 10⁻³ pH = -log(10⁻³) = -(-3) = 3
A solution has pH = 10. Find [H⁺].
pH = -log[H⁺] 10 = -log[H⁺] log[H⁺] = -10 [H⁺] = 10⁻¹⁰ M
At 25°C, a solution has pOH = 4. Find pH.
pH + pOH = 14 pH = 14 - 4 = 10 (Solution is basic)
Flashcards
Quick quiz
Q1.[H⁺] = 10⁻⁴ M. What is pH?
Q2.A solution has pH = 9. Is it acidic or basic?
Q3.pH + pOH at 25°C equals…
Q4.If [H⁺] increases, pH…
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Common mistakes
pH and [H⁺] increase together. — Correct: They're inversely related: higher [H⁺] → lower pH.
pH can be negative or greater than 14. — Correct: Typical aqueous solutions range 0–14 (though extreme solutions can fall outside).
Ignoring the negative sign in pH = −log[H⁺]. — Correct: The negative sign flips the log, so log(10⁻³) = −3 becomes pH = 3.
Forgetting that pH + pOH = 14 only at 25°C. — Correct: At other temperatures, the constant differs (not always 14).
FAQ
What does pH stand for?
'pH' means 'power of hydrogen'—a logarithmic scale of [H⁺] concentration.
How do I find [H⁺] if I know pH?
Use the inverse: [H⁺] = 10⁻ᵖᴴ. Example: pH 5 → [H⁺] = 10⁻⁵ = 0.00001 M.
Why use logarithms for pH?
[H⁺] spans 10 orders of magnitude; log scale compresses it to an easy 0–14 range.
What is [H⁺] in neutral water?
In neutral water at 25°C, [H⁺] = 10⁻⁷ M = 0.0000001 M, so pH = 7.