pH Formula

The pH formula measures the acidity or basicity of a solution on a scale from 0 to 14. pH is one of the most important measurements in chemistry, biology, environmental science, and medicine.

The pH Formula

pH = -log[H⁺]

Negative logarithm of hydrogen ion concentration

Variable Definitions

pH = Power of Hydrogen

Units: Unitless (dimensionless)

Range: 0 (most acidic) to 14 (most basic)

Neutral: pH = 7 at 25°C

[H⁺] = Hydrogen Ion Concentration

Units: M (mol/L or molar)

Meaning: Concentration of H⁺ ions in solution

💡 Also written as [H₃O⁺] (hydronium ion)

log = Logarithm (base 10)

The common logarithm, not natural log (ln)

The pH Scale

pH 0-6

ACIDIC

More H⁺ ions

pH 7

NEUTRAL

Equal H⁺ and OH⁻

pH 8-14

BASIC

More OH⁻ ions

Related Formulas

Find [H⁺] from pH

[H⁺] = 10-pH

Reverse calculation

pOH Formula

pOH = -log[OH⁻]

For bases

pH + pOH Relationship

pH + pOH = 14

At 25°C

Find pH from pOH

pH = 14 - pOH

Quick conversion

Step-by-Step Example

Problem: What is the pH of a solution with [H⁺] = 1.0 × 10⁻³ M?

Step 1: Write the formula

pH = -log[H⁺]

Step 2: Substitute the value

pH = -log(1.0 × 10⁻³)

Step 3: Calculate

pH = -(-3) = 3

Answer: pH = 3 (acidic solution)

Common Mistakes

❌ Forgetting the negative sign

It's -log[H⁺], not just log[H⁺]. The negative makes pH values positive.

❌ Using natural log (ln) instead of log

pH uses log base 10, not ln (natural log). Make sure your calculator is in log mode.

❌ Confusing acidic and basic

Lower pH = more acidic. Higher pH = more basic. pH 7 is neutral.

Related Calculators

pH Calculator

Calculate pH from [H⁺] or vice versa

Henderson-Hasselbalch

Calculate buffer pH

Titration Calculator

pH changes during titration

Buffer Capacity

Measure pH resistance

Frequently Asked Questions

What is the pH formula?

pH = -log[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. It measures acidity on a 0-14 scale.

Why is the pH scale 0 to 14?

At 25°C, water's ion product is 10⁻¹⁴. Since pH + pOH = 14, the practical range is 0-14, though values outside this range exist for very strong acids/bases.

How do I calculate pH without a calculator?

For powers of 10: If [H⁺] = 10⁻ⁿ, then pH = n. Example: [H⁺] = 10⁻⁵ → pH = 5.

What is the difference between pH and pOH?

pH measures [H⁺] while pOH measures [OH⁻]. They're related: pH + pOH = 14 at 25°C.

Why is pH 7 neutral?

Pure water has [H⁺] = [OH⁻] = 10⁻⁷ M at 25°C, giving pH = -log(10⁻⁷) = 7.

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