Calculate pH, pOH, hydronium or hydrogen ion concentration, and hydroxide concentration from one known value, with a 25 °C default and optional custom pKw.
Last updated
Known value
Quick presets
Assumptions
The default assumes aqueous conditions at 25 °C, where pH + pOH = 14 and the ion product of water is 1.0 × 10^-14. Custom pKw changes the neutral point but still treats concentrations as ideal.
Result
Neutral. Hydronium and hydroxide are equal at the selected neutral point of pH 7.
This is neutral at the selected pKw because pH equals pOH at 7.
pOH = pKw - pH = 14 - pH, then [H+] = 10^-pH and [OH-] = 10^-pOH.
| Reference solution | Typical pH | Context |
|---|---|---|
| Lemon juice | 2 | strongly acidic food range |
| Vinegar | 2.4–3.4 | acidic household example |
| Pure water at 25 °C | 7 | neutral when pKw is 14 |
| Blood | 7.35–7.45 | slightly basic biological range |
| Baking soda solution | ~8.3 | mildly basic |
| Household ammonia | 11–12 | basic cleaning-solution range |
Science — Chemistry
A pH calculator converts between pH, pOH, hydronium concentration, hydrogen ion concentration, and hydroxide concentration. Use it as a pOH calculator, a pH to H+ concentration calculator, or a concentration to pH calculator under the standard aqueous 25 °C convention, with an optional custom pKw when your problem supplies a different water-ion relation.
pH is a logarithmic way to describe the effective hydronium-ion level in an aqueous solution. Lower pH values indicate more acidic conditions, while higher values indicate more basic conditions.
Because the scale is logarithmic, a one-unit pH change represents a tenfold change in hydronium concentration. That is why small pH differences can correspond to large chemical differences in the solution itself.
For introductory aqueous chemistry at 25 °C, the standard relations are pH = -log10[H+], pOH = -log10[OH−], and pH + pOH = 14. Those relations come from the water-ion product Kw = 1.0 × 10^-14 under the same temperature assumption.
This calculator uses those relations consistently to derive the full four-value profile from whichever one value you already know.
pH = -log10([H+])
Converts hydronium concentration in mol/L into the pH scale.
pOH = -log10([OH−])
Converts hydroxide concentration in mol/L into the pOH scale.
pH + pOH = 14
Applies the standard 25 °C water-ion relation used in general chemistry.
Suppose the measured pH is 3.20. The calculator converts that pH into a hydronium concentration of about 6.31 × 10^-4 M, then uses Kw to derive the hydroxide concentration and the matching pOH.
That makes the page useful whether you start with a meter reading, a worksheet value, or a concentration given in molarity notation.
Many pH calculator pages hard-code pH + pOH = 14. That is the familiar 25 °C teaching convention, but the more general relationship is pH + pOH = pKw. When pKw changes, the neutral point is pKw ÷ 2 rather than always 7.
The custom pKw control is useful for chemistry problems that already provide a temperature-specific pKw. It should not be read as a full temperature model by itself, because real analytical pH work can also involve activity coefficients, calibration standards, ionic strength, and the measurement procedure.
pH + pOH = pKw
General water-ion relation. At 25 °C, pKw is commonly rounded to 14.00.
neutral pH = pKw / 2
Neutral means [H+] and [OH−] are equal under the selected water-ion model.
For a classroom or quick lab-prep check, enter whichever value is known: pH, pOH, [H+], or [OH−]. The result shows the complementary concentration, the acid-base band, the neutral point for the selected pKw, and a short explanation of whether hydronium or hydroxide is dominant.
For strong acid or strong base concentration problems, first decide whether the entered concentration really equals [H+] or [OH−]. A monoprotic strong acid such as HCl is often treated as [H+] = C in introductory problems, while a base with more than one hydroxide per formula unit may require a stoichiometric factor before using the pH converter.
This page is intentionally a pH, pOH, and ion-concentration converter. It does not solve weak acid equilibrium, weak base equilibrium, buffer pH, titration curves, polyprotic acid systems, or activity-corrected analytical pH.
If your problem gives Ka, Kb, pKa, pKb, buffer components, or a titration volume, you need an acid-base equilibrium setup before the pH number can be trusted. Use this page after that equilibrium step has produced the actual hydronium or hydroxide concentration.
Frequently asked questions
Because the pH scale is logarithmic. Moving from pH 7 to pH 6 means the hydronium concentration increases by a factor of ten, not by a small linear amount.
Yes. The classroom range of 0 to 14 is a common teaching simplification for many dilute aqueous solutions, but concentrated systems can fall outside that range. This calculator still uses the standard 25 °C Kw convention when it derives the related values.
Only as a simplified reference. It does not model ionic strength, activity corrections, non-aqueous solvents, or buffer equilibria. For formal analytical work, rely on calibrated measurement methods and the relevant laboratory procedure.
The logarithmic scale compresses very large concentration ranges into numbers that are easier to read and compare. Because each one-unit change reflects a tenfold change in hydronium concentration, pH is much easier to scan than a raw molarity value across acidic and basic solutions.
pH describes hydronium concentration and pOH describes hydroxide concentration. At 25 °C they add to 14, so knowing one lets you calculate the other under the standard aqueous assumption used by this calculator.
Use pH = -log10([H+]) with [H+] in mol/L. For example, [H+] = 1 × 10^-4 M gives pH 4. The calculator accepts M, mM, μM, and nM inputs and converts them to mol/L before applying the logarithm.
Use the inverse relationship [H+] = 10^-pH. A pH of 5 corresponds to [H+] = 1 × 10^-5 M, while a pH of 6 corresponds to 1 × 10^-6 M, so the lower pH has ten times more hydronium concentration.
pKw is the negative logarithm of Kw, the water-ion product. In the simplified relationship used here, pH + pOH = pKw. The standard classroom value is pKw = 14.00 at 25 °C, but some problems provide another pKw for a different temperature.
pH 7 is neutral under the 25 °C convention where pKw is 14. If a problem uses a different pKw, the neutral point becomes pKw ÷ 2 because neutral means [H+] and [OH−] are equal.
Not directly. Weak acid and weak base problems require an equilibrium calculation using Ka, Kb, pKa, or pKb before [H+] or [OH−] is known. Once that concentration is known, this page can convert it into pH, pOH, and the complementary ion concentration.
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