pH Calculator

pH measures the acidity or basicity of a solution on a scale from 0-14. pH = -log[H⁺]. Acidic solutions have pH < 7, neutral pH = 7, and basic solutions have pH > 7. Each pH unit represents a 10-fold change in [H⁺].

🔬 Acid-Base📐 pH = -log[H⁺]🧪 General Chemistry

Hydrogen ion concentration [H⁺] (mol/L)

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
pH	pH	-log[H⁺]	0–14
[H⁺]	[H⁺]	10^-pH	mol/L
pOH	pOH	-log[OH^-]	0–14
Water at 25°C	—	pH + pOH = 14	—
[H⁺][OH⁻]	Kw	1.0 × 10^-14 at 25°C	(mol/L)²

Step-by-Step Examples

Example 1

[H⁺] to pH

Find pH of solution with [H⁺] = 3.50×10⁻⁴ mol/L.

pH = -log(3.50×10⁻⁴)
pH = -(-3.456) = 3.456

✓ pH = 3.46 (acidic)

Example 2

pH to [H⁺]

Find [H⁺] when pH = 8.25.

[H⁺] = 10⁻⁸·²⁵
[H⁺] = 5.62×10⁻⁹ mol/L

✓ [H⁺] = 5.62×10⁻⁹ mol/L, basic solution

Example 3

From pOH

pOH = 4.75 at 25°C.

pH = 14 - 4.75 = 9.25
[H⁺] = 10⁻⁹·²⁵ = 5.62×10⁻¹⁰ mol/L

✓ pH = 9.25 (basic)

Real-World Applications

💊

Medicine

Blood pH = 7.35-7.45. Small deviations cause acidosis or alkalosis.

🌊

Environmental Science

Ocean acidification (pH dropping from 8.2 to 8.1) threatens marine ecosystems.

🧪

Lab Chemistry

Reaction rates, solubility, and enzyme activity all depend critically on pH.

🍿

Food Science

Beer pH 4-5, milk pH 6.5, cola pH 2.5. pH affects flavor and preservation.

Common Mistakes to Avoid

⚠️

Forgetting the negative sign

pH = -log[H⁺]. The negative sign makes pH positive for typical [H⁺] values. [H⁺] = 0.01 M gives pH = -log(0.01) = 2.

⚠️

pH + pOH = 14 only at 25°C

Kw changes with temperature. At 37°C (body temp), Kw = 2.4×10⁻¹⁴, so pH + pOH = 13.62.

⚠️

pH scale goes beyond 0-14

Concentrated strong acid can give pH < 0 (e.g., pH = -0.3 for 2 M HCl). Strong base can give pH > 14.

Frequently Asked Questions

Why is pH on a logarithmic scale? ▾

[H⁺] spans 14 orders of magnitude (10⁻¹⁴ to 1 mol/L). Logarithmic scale compresses this to 0-14 for convenience.

What is the pH of pure water? ▾

At 25°C: [H⁺] = [OH⁻] = 1.0×10⁻⁷ M, pH = 7.00. At 37°C: pH = 6.81 (still neutral, but Kw is larger).

How does pH relate to acidity? ▾

Lower pH = more acidic = higher [H⁺]. Each pH unit = 10x change in [H⁺]. pH 3 is 100x more acidic than pH 5.

What pH is stomach acid? ▾

Stomach acid (HCl) has pH 1.5-2.0. [H⁺] = 0.03-0.10 M. This is necessary for pepsin enzyme activity and pathogen killing.

How are pH meters calibrated? ▾

Using pH buffer solutions (pH 4.00, 7.00, 10.00). The electrode voltage changes 59.16 mV per pH unit at 25°C (Nernst equation).

Related Chemistry Calculators

Formula Explorer connections

Interpretation: pH is logarithmic, so a decrease of one pH unit means ten times greater hydrogen-ion activity. Assumption: concentration approximates activity in dilute solutions; pH+pOH=14 applies specifically at 25°C.

pOH →Acid Dissociation →Buffer pH →Chemistry Formula Explorer →