Calculator Form
Use any supported starting value. Concentration inputs must be entered in mol/L.
Formula Used
The calculator uses standard aqueous relationships between pH, pOH, and the ion product of water.
Kw = 10^(-pKw)pH = -log10([H⁺])pOH = -log10([OH⁻])pH + pOH = pKw[OH⁻] = 10^(-pOH)[H⁺] = 10^(-pH)[OH⁻] = Kw / [H⁺]OH⁻ moles = [OH⁻] × volume (L)
For mass estimates, the page uses the molar mass of hydroxide ion as 17.00734 g/mol and sodium hydroxide as 39.997 g/mol.
Neutral reference pH is calculated as pKw / 2, which is useful when you use custom pKw values.
How to Use This Calculator
- Choose whether your known value is pH, pOH, hydroxide concentration, or hydrogen concentration.
- Enter the numeric value for the selected input mode.
- Set pKw. Keep 14.00 for common room-temperature textbook work unless your problem states otherwise.
- Optionally enter solution volume to estimate total hydroxide moles and related mass values.
- Choose significant figures and press the calculate button to show the result, graph, and export tools.
Example Data Table
| Input Mode | Entered Value | pKw | Calculated pH | Calculated pOH | [OH⁻] (mol/L) | Solution Type |
|---|---|---|---|---|---|---|
| pH | 11.30 | 14.00 | 11.3000 | 2.7000 | 1.995e-03 | Basic |
| pOH | 4.20 | 14.00 | 9.8000 | 4.2000 | 6.310e-05 | Basic |
| [H⁺] | 1.00e-09 | 14.00 | 9.0000 | 5.0000 | 1.000e-05 | Basic |
| [OH⁻] | 2.50e-02 | 14.00 | 12.3979 | 1.6021 | 2.500e-02 | Basic |
FAQs
1. What does this hydroxide ion calculator compute?
It converts among pH, pOH, hydroxide concentration, and hydrogen concentration using the ion product of water. Optional volume also estimates hydroxide moles and related mass values.
2. Why is pKw editable?
The common 25°C assumption uses pKw = 14.00, but temperature or study conventions can change it. Editing pKw lets the calculator match your chemistry context.
3. When should I enter [H⁺] instead of pH?
Use [H⁺] when a lab report or textbook problem provides molar hydrogen concentration directly. The calculator then handles the logarithms automatically.
4. What units should I use for concentration?
Use mol/L, also written as M. Convert millimolar or micromolar values into mol/L before entering them for correct pH and pOH results.
5. Why does an acidic solution still show a hydroxide value?
Water autoionization means hydroxide is rarely exactly zero in aqueous systems. Acidic solutions simply contain much less hydroxide than hydrogen ions.
6. Does volume change pH or pOH in this tool?
No. Volume only affects total moles and mass estimates after concentration is known. It does not change pH or pOH by itself.
7. Can this calculator be used for weak bases?
Yes, if you already know the final equilibrium concentration. First solve the weak-base equilibrium, then enter the resulting pH, pOH, [H⁺], or [OH⁻] here.
8. Is this a substitute for direct lab measurement?
No. It is a calculation aid. Real samples may differ because of temperature, activity effects, contamination, ionic strength, or instrument calibration limits.