pH measures how acidic or basic a solution is. Enter the hydrogen-ion concentration or the pH to get pH, pOH and both ion concentrations at 25 °C.
Choose whether you know [H⁺] or pH.
Enter the value.
Read pH, pOH and concentrations.
pH: pH = −log₁₀[H⁺]
pOH: pOH = 14 − pH
pH is a compact number that says how acidic or basic a watery solution is. This calculator works in either direction. Give it the hydrogen-ion concentration [H⁺] in mol/L and it finds the pH; give it the pH and it finds [H⁺]. In both cases it also returns pOH and the hydroxide-ion concentration [OH⁻], and labels the solution acidic, neutral or basic, all at 25 °C.
Class 10 science introduces the pH scale with indicators and everyday examples, and Class 11 chemistry puts numbers to it through ionic equilibrium. The same scale matters well beyond exams: in checking drinking water, testing farm soil at a Krishi Vigyan Kendra, maintaining swimming pools and aquariums, and controlling food and pharmaceutical processes.
1. Find [H⁺] in mol/L. For a strong monoprotic acid like HCl, [H⁺] equals the acid's molarity.
2. Take the base-10 logarithm of [H⁺] and change its sign: pH = −log₁₀[H⁺].
3. Find pOH from pOH = 14 − pH, valid at 25 °C.
4. Find [OH⁻] = 10^(−pOH).
5. To go the other way, start from pH and use [H⁺] = 10^(−pH).
6. Classify: pH below 7 is acidic, 7 is neutral and above 7 is basic, at 25 °C.
Hydrogen-ion concentrations in water span from about 1 mol/L in strong acid to 10⁻¹⁴ mol/L in strong base. Writing such numbers is awkward, so Sørensen defined pH as the negative logarithm. Each step of one pH unit is a tenfold change in [H⁺]. Lemon juice at pH 2 has about 100,000 times more hydrogen ions than water at pH 7. The negative sign makes the everyday range positive, but it also means a lower pH is more acidic.
Water ionises slightly: H₂O ⇌ H⁺ + OH⁻. At 25 °C the product of the two concentrations, the ionic product Kw = [H⁺][OH⁻], is 1.0 × 10⁻¹⁴. Taking −log₁₀ of both sides gives pH + pOH = 14. In pure water [H⁺] = [OH⁻] = 10⁻⁷, so pH is 7. Kw grows with temperature, so at higher temperatures neutral water has a pH below 7 while still being neutral. The calculator uses the 25 °C value throughout.
For a strong acid, all the acid ionises, so [H⁺] equals its concentration, and 0.01 M HCl has pH 2. A weak acid like acetic acid ionises only partly, so its [H⁺] is much less than its concentration and must first be found from its Ka. Very dilute acids also need care: 10⁻⁸ M HCl is not basic, because water's own H⁺ then matters. In real labs, pH meters measure hydrogen-ion activity, which differs from concentration in concentrated solutions.
Farmer Ramesh's soil-test report from the district lab says his field's soil water has a pH of 5.6, and he wants the matching hydrogen-ion concentration.
[H⁺] = 10^(−pH): = 10^(−5.6) = 2.5119 × 10^-6 mol/L
pOH = 14 − pH (at 25 °C): = 14 − 5.6 = 8.4
[OH⁻] = 10^(−pOH): = 3.9811 × 10^-9 mol/L
Answer: pH 5.6; pOH 8.4; [H⁺] 2.5119 × 10^-6 mol/L
Using the natural logarithm ln instead of log₁₀.
Forgetting the minus sign and getting a negative pH for ordinary acids.
Treating a weak acid's molarity as its [H⁺], which makes it look far more acidic than it is.
Thinking pH 4 is twice as acidic as pH 8, when it has 10,000 times more H⁺.
Using pH + pOH = 14 at temperatures far from 25 °C.
Class 10 to 12 chemistry problems and JEE and NEET ionic equilibrium questions.
Interpreting soil test results before choosing lime or fertiliser.
Checking drinking water, swimming pools and aquariums.
Controlling pH in food processing, dairy and pharmaceutical manufacturing.
Converting between pH meter readings and ion concentrations in lab reports.
Can pH be below 0?
Yes, for very concentrated strong acids, though school problems stay within 0–14.
What is the pH of 0.01 M HCl?
HCl is a strong acid, so [H⁺] = 0.01 and pH = 2.