What is pH Calculation?
pH measures how acidic or basic a solution is, based on the concentration of hydrogen ions dissolved in it. It runs from 0 (strongly acidic) to 14 (strongly basic), with 7 as neutral.
pH is calculated as pH = -log[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L; lower pH means a higher H⁺ concentration and a stronger acid.
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Step-by-step worked examples
A solution has [H⁺] = 1 × 10⁻³ mol/L. Find its pH.
pH = -log[H⁺] pH = -log(0.001) = -log(10⁻³) pH = 3
Pure water has [H⁺] = 1 × 10⁻⁷ mol/L. Find its pH and state whether it is acidic, basic or neutral.
pH = -log(10⁻⁷) = 7 pH = 7 → neutral
A solution has a pH of 4. Find its hydrogen ion concentration [H⁺].
pH = -log[H⁺] → [H⁺] = 10^(-pH) [H⁺] = 10⁻⁴ [H⁺] = 0.0001 mol/L
Flashcards
Quick quiz
Q1.What is the pH of a solution with [H⁺] = 1 × 10⁻⁵ mol/L?
Q2.A solution has pH 2. Which best describes it?
Q3.If [H⁺] increases, what happens to pH?
Q4.What is [H⁺] for a solution with pH 6?
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Common mistakes
Thinking a higher pH means a higher [H⁺]. — Correct: Higher pH means LOWER [H⁺] — the relationship is inverse and logarithmic.
Forgetting the negative sign in pH = -log[H⁺]. — Correct: Since [H⁺] is usually less than 1, log[H⁺] is negative — the minus sign makes pH positive.
Treating pH changes as linear. — Correct: Each whole pH unit represents a 10× change in [H⁺] — it's a logarithmic scale.
Forgetting units — writing [H⁺] without mol/L. — Correct: [H⁺] must be expressed in mol/L (molarity) before taking the log.
FAQ
What is pH calculation?
pH calculation is finding how acidic or basic a solution is using the formula pH = -log[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L.
What is the pH formula?
pH = -log[H⁺]. To reverse it, use [H⁺] = 10^(-pH).
What are examples of pH calculation?
Pure water ([H⁺]=10⁻⁷) has pH 7; lemon juice ([H⁺]≈10⁻²) has pH about 2; ammonia solution has a pH around 11.
How do you calculate pH from concentration?
Take the hydrogen ion concentration in mol/L, apply pH = -log[H⁺], and round to the appropriate number of decimal places.