Case 1
- Hydrogen-ion concentration [H⁺]
- 1e-7mol/L
7.00
Open with these valuespH Calculator
7.00pH
Result: 7.00 pHpH is the negative base-10 logarithm of the hydrogen-ion concentration: pH = −log₁₀([H⁺]), with [H⁺] in mol/L. Neutral water is 1 × 10⁻⁷ mol/L and gives pH 7. Each whole pH unit is a tenfold change in [H⁺], and pOH = 14 − pH holds in dilute solution at 25 °C.
Worked examples
Case 2
- Hydrogen-ion concentration [H⁺]
- 0.001mol/L
3.00
Open with these valuesCase 3
- Hydrogen-ion concentration [H⁺]
- 0.0000316mol/L
4.50
Open with these valuesHow it's calculated
pH = −log₁₀([H⁺])
- StepEnter the hydrogen-ion concentration in moles per litre.
- StepRead the pH — below 7 acidic, 7 neutral, above 7 basic.
- ResultFor the pOH at 25 °C, subtract the pH from 14.
What this number means
One pH unit is a factor of ten
The scale is logarithmic, so pH 5 is not slightly more acidic than pH 6 — it is ten times. Two steps, as from pH 3 to pH 5, is a hundredfold change in [H⁺].
pOH = 14 − pH holds at 25 °C
That 14 is the ion product of water, a measured quantity that shifts with temperature. At 0.001 mol/L the pH is 3 and the pOH 11.
Strictly the activity, not the concentration
The pH is defined over the activity of the hydrogen ions. In dilute solution activity and concentration coincide; in concentrated solution they do not.
Enter mol/L, and never zero
The field takes moles per litre, from 1 × 10⁻¹⁴ up to 10. Zero is ruled out because the logarithm of zero is undefined.
Commonly misread
pH 5 is a little more acidic than pH 6.
It is ten times more acidic. Each whole pH unit is a tenfold change in [H⁺].
pOH = 14 − pH is a universal rule.
It holds in dilute aqueous solution at 25 °C. The 14 is the ion product of water and shifts with temperature.
A concentration of 0 mol/L gives pH 0.
The logarithm of zero is undefined, so the field starts at 1 × 10⁻¹⁴ mol/L. pH 0 belongs to 1 mol/L instead.
Reference table
| [H⁺] in mol/L | pOH at 25 °C | pH |
|---|---|---|
| 0.1 | 13 | 1.00 |
| 0.001 | 11 | 3.00 |
| 0.0001 | 10 | 4.00 |
| 0.0000316 | 9.50 | 4.50 |
| 0.0000001 | 7 | 7.00 |
| 0.00000000001 | 3 | 11.00 |
Questions
How do I calculate pH?
Take the negative base-10 logarithm of the hydrogen-ion concentration: pH = −log₁₀([H⁺]), with [H⁺] in moles per litre. A solution at 0.001 mol/L has pH 3, and its pOH is 14 − 3 = 11.
What is pH?
pH measures how acidic or basic a water-based solution is, usually on a scale from 0 to 14. A pH of 7 is neutral, below 7 is acidic and above 7 is basic.
Why is the pH scale logarithmic?
Because hydrogen-ion concentrations span roughly fourteen orders of magnitude, from about 1 mol/L down to 10⁻¹⁴ mol/L. The base-10 logarithm compresses that into a readable scale, so each whole pH unit is a tenfold change in [H⁺].
What is the difference between pH and pOH?
pH measures the hydrogen-ion concentration, pOH the hydroxide-ion concentration. In dilute aqueous solution at 25 °C they add up to 14, so pOH = 14 − pH. That 14 is the ion product of water and shifts with temperature, so the rule is not universal.
Which unit does the concentration use?
Moles per litre. Pure neutral water is 1 × 10⁻⁷ mol/L, which gives pH 7, and the concentration must be greater than zero because the logarithm of zero is undefined. The pH itself carries no unit.
Sources and last check
Information, not professional advice.