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Chemistry formula

pH formula:
story, analogy and memory trick

The pH formula is pH = −log₁₀[H⁺]. pH is the negative base-10 logarithm of the hydrogen-ion concentration, so a lower pH means a more acidic solution.

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The pH formula

pH = −log₁₀[H⁺]
  • pHmeasure of acidity
  • [H⁺]hydrogen-ion concentration, in mol/L

The story

Imagine H⁺ ions as guests at a party. When the hall is packed, the host takes the negative log and reports a small number, so pH 1 means a crowded, strongly acidic party. An almost empty hall gives a large number, such as pH 13.

Real-life analogy

Think of the Richter scale for earthquakes. Each step is a tenfold change, so pH 3 has ten times more H⁺ than pH 4.

Memory trick

The letter p stands for the power of ten. If [H⁺] = 10⁻³ mol/L, the power is −3, so pH = 3.

Exam tip

Each pH unit is a tenfold change in [H⁺]. At 25 °C, pH 7 is neutral and pH + pOH = 14.

FAQ

Questions about pH

What is the pH formula?
pH = −log₁₀[H⁺]. pH is the negative base-10 logarithm of the hydrogen-ion concentration, so a lower pH means a more acidic solution.
How do I remember the pH formula?
The letter p stands for the power of ten. If [H⁺] = 10⁻³ mol/L, the power is −3, so pH = 3.
What should I watch out for in exams with pH?
Each pH unit is a tenfold change in [H⁺]. At 25 °C, pH 7 is neutral and pH + pOH = 14.

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