pH Calculator

What you know

H⁺ concentration, mol/l

Result

pH3
Character of the solutionacidic
pOH11
H⁺ concentration, mol/l0.001
OH⁻ concentration, mol/l0
Degree of dissociation, %weak acid mode only

A logarithmic scale defeats intuition

Works out pH and pOH from a hydrogen ion concentration or from the dissociation constant of a weak acid.

pH is the negative base-ten logarithm of the hydrogen ion concentration. The logarithm makes the scale deceptive: the gap between pH 3 and pH 4 is not 'a little', it is tenfold in acidity, and between pH 3 and pH 6 it is a thousandfold.

Strong acids are straightforward: they dissociate completely, so the ion concentration equals the acid concentration. Weak acids dissociate partially, and for those the approximation runs through the constant — the ion concentration is the square root of the constant times the concentration.

What this calculator shows

pH and pOH, which always sum to 14
Hydrogen and hydroxide ion concentrations
The degree of dissociation in weak-acid mode

What to keep in mind

pH plus pOH equals 14 at 25 °C; at other temperatures the ion product of water shifts.
The weak-acid approximation holds while dissociation stays under about five percent.

FAQs

How do I calculate pH?

Take the negative base-ten logarithm of the hydrogen ion concentration. For 0.001 mol/l that gives pH 3.

How are pH and pOH related?

They sum to 14 at 25 degrees. pH 3 means pOH 11 and a hydroxide concentration of 10⁻¹¹ mol/l.

What separates a strong acid from a weak one?

A strong acid dissociates completely, so ion concentration equals acid concentration. A weak acid dissociates by a fraction of a percent, and its pH runs through Ka.

Can pH go below zero?

Yes, in concentrated acids. The 0 to 14 range is a convenient span for aqueous solutions, not a physical limit.

Worked example

Acetic acid

Input: Ka = 1.75 × 10⁻⁵, concentration 0.1 mol/l

Output: pH 2.88, dissociation 1.32%

Note: Hydrochloric acid at the same concentration would give pH exactly 1 — nearly eighty times more hydrogen ions. That gap is the whole difference between a strong and a weak acid.

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pH Calculator

Result

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