Convert between pH, pOH, [H₃O⁺], and [OH⁻]
pH and pOH are base-10 logarithmic expressions of hydronium and hydroxide quantities. For the common introductory approximation of a dilute aqueous solution at 25 °C, pKw is 14.00.
The ion product of water is temperature dependent. This calculator exposes pKw so a value appropriate to other conditions can be entered instead of silently assuming 14 in every case.
How to use the pH and pOH calculator
- Choose a known quantity: Select pH, pOH, [H₃O⁺], or [OH⁻].
- Enter its value: Concentrations must be positive.
- Set pKw: Keep 14.00 for the standard 25 °C classroom approximation or supply another justified value.
- Calculate: Review all four related acid–base quantities.
Formula and variables
Apply the base-10 logarithm to ion concentration, then use pKw to obtain the complementary value.
pH = −log₁₀[H₃O⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = pKw- pH — pH
- Negative base-10 logarithm of hydronium
- pOH — pOH
- Negative base-10 logarithm of hydroxide
- [H₃O⁺] — Hydronium concentration
- Hydronium molarity under the concentration approximation (mol/L)
- [OH⁻] — Hydroxide concentration
- Hydroxide molarity under the concentration approximation (mol/L)
Convert pH to pOH and ion concentrations
A dilute aqueous solution has pH 5.00 at 25 °C.
- pH
- 5.00
- pKw
- 14.00
- pOH = 14.00 − 5.00 = 9.00
- [H₃O⁺] = 10⁻⁵ M
- [OH⁻] = 10⁻⁹ M
Result: pOH is 9.00, [H₃O⁺] is 1.0 × 10⁻⁵ M, and [OH⁻] is 1.0 × 10⁻⁹ M.
At 25 °C this solution is acidic because its hydronium exceeds hydroxide.
Understanding your results
Temperature and activity matter
Neutral means equal hydronium and hydroxide activities; it does not always mean pH 7.00 because pKw changes with temperature.
- At pKw 14.00, neutrality corresponds to pH 7.00.
- Very concentrated or nonideal solutions require activity-based treatment.
Assumptions
- The entered pKw applies to the solution temperature and conditions.
- Concentration is an acceptable approximation for activity.
- Base-10 logarithms are used.
Limitations
- Does not calculate activity coefficients or ionic-strength corrections.
- Does not infer pKw from temperature.
- Does not model acid dissociation, buffers, titrations, or mixtures.
Common mistakes
- Using pH + pOH = 14 without checking temperature.
- Entering zero or a negative concentration.
- Using a natural logarithm instead of base 10.
- Assuming pH must always lie between 0 and 14.
Practical use cases
General chemistry
Practice logarithmic acid–base conversions.
Result checks
Check whether complementary pH and pOH values agree with the selected pKw.
Frequently asked questions
Why does the calculator let me change pKw?
The ion product of water changes with temperature and solution conditions. A fixed value of 14.00 is the common dilute-water approximation at 25 °C.
Can pH be below 0 or above 14?
Yes in some concentrated solutions, although concentration-only formulas become less reliable as nonideality increases.
Is [H⁺] the same as [H₃O⁺]?
Introductory calculations often use H⁺ as shorthand, while hydronium H₃O⁺ better represents a proton solvated in water.
Sources and review
- pH and pOH — OpenStax Chemistry 2e. Accessed 2026-07-13.
Reviewed 2026-07-13.