pH is define as

Analyze the question: You're asked for the definition of pH, which measures how acidic or basic a solution is.

Concept: pH was defined by Sørensen as the negative base-10 logarithm of the hydrogen ion concentration (in mol/dm³):

pH = −log₁₀ [H⁺]

Step by step:

1. Option A: [H⁺] is just the hydrogen ion concentration, not pH.
2. Option B: −log [H] uses [H], which refers to hydrogen atoms or molecules, not the hydrogen ion H⁺. This is incorrect.
3. Option C: −log [H⁺] is the correct definition, with the negative sign and the hydrogen ion concentration.
4. Option D: log [H] has no negative sign and uses [H] instead of [H⁺]. Incorrect.
5. Option E: log [H⁺] is missing the negative sign. It would give negative values for most acidic solutions, so it is incorrect.

Quick example: If [H⁺] = 1 × 10⁻³ mol/dm³, then pH = −log(10⁻³) = 3, so the solution is acidic.

Exam tip: Remember "pH = Negative Log of H-plus." The negative sign is what makes pH values positive for most common solutions. Also, a higher [H⁺] means a lower pH, so the two move in opposite directions.