pH Calculator
Convert between pH, pOH, [H⁺] and [OH⁻] for aqueous solutions at 25 °C.
How to use the pH Calculator
- Choose the known value.
- Enter the value – Scientific notation is allowed, e.g. 1e-7 or 2.5e-3.
- Press Calculate. The result updates automatically as you type.
About the pH Calculator
The pH scale expresses how acidic or basic a water-based solution is. Because it is logarithmic, each whole pH unit is a tenfold change in hydrogen ion concentration: a solution at pH 3 is ten times more acidic than one at pH 4 and a thousand times more acidic than pure water at pH 7.
For a strong acid such as HCl, [H⁺] equals the acid concentration, so 0.01 M HCl has a pH of 2. For a strong base such as NaOH, [OH⁻] equals the base concentration. Weak acids and bases only partly dissociate and need their Ka or Kb to find [H⁺]. The relationship pH + pOH = 14 holds at 25 °C; at other temperatures the water ion product changes slightly.
Formula
pH = −log₁₀[H⁺]
pOH = −log₁₀[OH⁻]
pH + pOH = 14 (at 25 °C)
[H⁺] = 10−pH
Concentrations are in mol/L. Example: [H⁺] = 3.2 × 10⁻⁴ M gives pH = −log(0.00032) ≈ 3.49, which is acidic, similar to orange juice or soda.
Example calculation
Using the default values:
- Known value = [H⁺] concentration (mol/L)
- Value = 3.2e-4
the calculator returns:
- pH: 3.495
- pOH: 10.505
- [H⁺]: 3.2 × 10^-4 mol/L
- [OH⁻]: 3.125 × 10^-11 mol/L
- Solution is: Acidic
Frequently asked questions
What is the pH of pure water?
Exactly 7 at 25 °C, where [H⁺] = [OH⁻] = 1 × 10⁻⁷ mol/L. At higher temperatures neutral pH drops slightly, for example about 6.14 at 100 °C.
Can pH be negative or above 14?
Yes. Very concentrated strong acids can have pH below 0, and very concentrated strong bases above 14.
What is the pH of 0.1 M NaOH?
[OH⁻] = 0.1 M, so pOH = 1 and pH = 14 − 1 = 13.
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