Enter solution details
Plotly graph
The chart compares hydrogen and hydroxide concentrations on a log scale and places pH and pOH on a second axis.
Formula used
Core relationships
pH = -log10[H+]
pOH = -log10[OH-]
pH + pOH = pKw
Kw = [H+][OH-] = 10-pKw
Submit a calculation to see the exact mode-specific formula and worked expression.
How to use this calculator
- Select the calculation mode that matches your known input.
- Enter pKw if your temperature differs from the standard condition.
- Fill only the fields needed for your chosen mode.
- Press the calculate button to display the result above the form.
- Review the graph, formula summary, and export buttons for documentation.
Example data table
| Example solution | Known input | pH | [H+] (mol/L) | [OH-] (mol/L) | Class |
|---|---|---|---|---|---|
| Pure water at 25°C | pH = 7.00 | 7.000000 | 1.000000e-7 | 1.000000e-7 | Neutral |
| Lemon juice sample | pH = 2.30 | 2.300000 | 5.011872e-3 | 1.995262e-12 | Acidic |
| Acetic acid estimate | C = 0.10 mol/L, Ka = 1.8e-5 | 2.875278 | 1.332160e-3 | 7.506544e-12 | Acidic |
| Sodium hydroxide solution | [OH-] = 0.010 mol/L | 12.000000 | 1.000000e-12 | 0.010000 | Basic |
Recent calculation log
No calculations have been stored yet.
FAQs
1. What does this calculator determine?
It calculates hydrogen ion concentration, pH, pOH, hydroxide concentration, and basic solution class. It also supports strong and weak acid or base estimation modes.
2. Can I start with pH or pOH?
Yes. Choose the matching mode, enter pH or pOH, and the tool converts that value into [H+], [OH-], and the paired logarithmic measure.
3. Why is pKw adjustable?
pKw changes with temperature. Allowing manual pKw entry makes the calculator more flexible for classroom exercises, lab work, and nonstandard solution conditions.
4. When should I use strong acid mode?
Use strong acid mode when dissociation is effectively complete, such as HCl or HNO₃ in dilute aqueous solutions. Enter molarity and the number of acidic protons released.
5. How does weak acid mode estimate [H+]?
It solves the quadratic form of the equilibrium expression using Ka and initial concentration. This avoids the rough small-x shortcut and improves accuracy for moderate dissociation cases.
6. Why are some answers shown in scientific notation?
Hydrogen and hydroxide concentrations often become extremely small or large. Scientific notation keeps those values readable and prevents long strings of zeros from hiding the scale.
7. Can the calculator handle very acidic or very basic solutions?
Yes. It can display negative pH or pH above 14 when the chosen concentration and pKw imply those results. Those outcomes are possible in concentrated systems.
8. What does the graph show?
The graph compares [H+] and [OH-] on a logarithmic concentration axis and overlays pH and pOH on a second axis for quick visual interpretation.