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pH Calculator

Convert between pH, pOH, [H⁺] and [OH⁻] for aqueous solutions at 25 °C.

pOH

[H⁺] (mol/L)

1.000e-3

[OH⁻] (mol/L)

1.000e-11

Nature

Acidic

Step-by-step working

Convert input to [H⁺] (using Kw = 1.0 × 10⁻¹⁴ at 25 °C if needed)
pH = −log₁₀[H⁺]; pOH = 14 − pH

Mathematical Formula

pH = −log₁₀[H⁺]; pOH = 14 − pH; [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ at 25 °C.

Overview & Explanation

pH expresses hydrogen-ion activity on a log scale; for dilute solutions concentration approximates activity.

How It Works

Convert the input to [H⁺].
Take −log₁₀.
Derive pOH and [OH⁻] using Kw.

Practical Applications

Chemistry labs.
Pool and aquarium water.

The pH formula

pH = -log10[H+], where [H+] is the hydrogen ion concentration in mol/L. A concentration of 0.001 M gives pH 3. A concentration of 2.5 x 10^-4 M gives -log10(0.00025) = 3.60.

Because the scale is logarithmic, each whole unit is a tenfold change. A solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4 and a hundred times that of pH 5.

pOH and bases

At 25 degrees C, pH + pOH = 14, where pOH = -log10[OH-]. A 0.01 M sodium hydroxide solution has [OH-] = 0.01, so pOH = 2 and pH = 12. The sum of 14 holds at 25 degrees C and shifts slightly at other temperatures.

Example 1: [H⁺] = 0.001 M

pH 3.

Inputs

I know: [H⁺] (mol/L) · Value: 0.001

= entered

Step 1

Convert input to [H⁺] (using Kw = 1.0 × 10⁻¹⁴ at 25 °C if needed)

Step 2

pH = −log₁₀[H⁺]; pOH = 14 − pH

pH

Result

= 3

Example 2: [OH⁻] = 0.01 M

pH 12.

Inputs

I know: [OH⁻] (mol/L) · Value: 0.01

= entered

Step 1

Convert input to [H⁺] (using Kw = 1.0 × 10⁻¹⁴ at 25 °C if needed)

Step 2

pH = −log₁₀[H⁺]; pOH = 14 − pH

pH

Result

= 12

How it's calculated & sources

pH = −log₁₀[H⁺]; pOH = 14 − pH; [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ at 25 °C.

IUPAC Gold Book – pH

Frequently Asked Questions

Is pH + pOH always 14?

Only at 25 °C, where Kw = 1.0 × 10⁻¹⁴.

Can pH be negative?

Yes, for very concentrated strong acids (> 1 M).

Strong vs weak acids?

This tool uses the given ion concentration directly; for weak acids compute [H⁺] from Ka first.

Can pH be below 0 or above 14?

Yes, for very concentrated strong acids or bases, though everyday solutions fall between 0 and 14.

Is this accurate for weak acids?

For weak acids, [H+] is not equal to the acid concentration, because only part of the acid dissociates. You first need the acid dissociation constant (Ka) to find [H+].

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