pH Calculator
Enter the hydrogen ion concentration [H⁺] in mol/L to find pH, pOH and [OH⁻].
pH measures acidity as the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺]. A concentration of 1×10⁻⁷ mol/L gives a neutral pH of 7. pOH is 14 − pH at 25°C, and the hydroxide concentration [OH⁻] is 10 raised to the negative pOH.
How to Use This Calculator
Enter the hydrogen ion concentration, [H⁺], in moles per litre. You can use scientific notation such as 1e-7 for 1×10⁻⁷. The calculator returns the pH, the pOH, and the hydroxide ion concentration [OH⁻].
pH runs from below 0 for very strong acids to above 14 for very strong bases, with 7 neutral at room temperature. Each whole pH unit is a tenfold change in acidity.
How the Calculation Works
pH is defined as the negative base-10 logarithm of the hydrogen ion concentration. The logarithm compresses the enormous range of concentrations — from about 1 mol/L down to 10⁻¹⁴ — into the familiar 0–14 scale.
At 25°C, pH and pOH always add to 14, because water self-ionizes with an ion product of 10⁻¹⁴. So once pH is known, pOH is 14 minus it, and the hydroxide concentration is 10 to the power of negative pOH.
pH = −log₁₀[H⁺]; pOH = 14 − pH; [OH⁻] = 10^(−pOH)
Worked Example
For [H⁺] = 1×10⁻⁷ mol/L: pH = −log₁₀(1×10⁻⁷) = 7, which is neutral. Then pOH = 14 − 7 = 7, and [OH⁻] = 10⁻⁷ mol/L.
For a stronger acid at [H⁺] = 0.01 mol/L: pH = −log₁₀(0.01) = 2, a clearly acidic solution.
Assumptions and Limitations
- Uses [H⁺] in mol/L; pH = −log₁₀[H⁺]
- The pH + pOH = 14 relationship assumes 25°C
- Treats concentration as activity (fine for dilute solutions)
- Very concentrated solutions deviate from this simple model
Who Uses a pH Calculator
Chemistry students use this to find pH, pOH, and hydroxide concentration from a hydrogen-ion concentration. Run it for acid-base homework, lab preparation, or to understand how these quantities relate. Enter the hydrogen-ion concentration to get the full set.
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Open calculator →Frequently Asked Questions
How do you calculate pH?
Take the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺]. For [H⁺] = 1×10⁻³ mol/L, pH = 3.
What is a neutral pH?
7 at 25°C, corresponding to [H⁺] = 1×10⁻⁷ mol/L. Below 7 is acidic and above 7 is basic. The neutral point shifts slightly with temperature.
How are pH and pOH related?
At 25°C they sum to 14, because water's ion product is 10⁻¹⁴. So pOH = 14 − pH, and you can find one from the other.
What does each pH unit mean?
Each whole unit is a tenfold change in hydrogen ion concentration. A pH 3 solution is ten times more acidic than pH 4 and a hundred times more than pH 5.
Can pH be negative or above 14?
Yes. Very concentrated strong acids can have a pH below 0, and very concentrated strong bases above 14. The 0–14 range simply covers most everyday solutions.
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