🔬 pH Calculator — Strong Acid
Calculate pH from the molar concentration of a strong acid. Applies to HCl, HBr, HNO₃, HClO₄, etc.
Enter molarity (M). Range: 1×10⁻¹⁴ to 10 M
How to Use This Calculator
Enter the molar concentration of your strong acid in mol/L (the same as M), and the calculator returns the pH, pOH, hydroxide concentration, and a label telling you whether the solution is acidic, basic, or neutral. This applies to any acid that dissociates 100% in water.
Identify your strong acid: HCl, HBr, HI, HNO₃, HClO₄, or H₂SO₄ (first proton). All of these dissociate completely.
Enter the molar concentration. If you dissolved 3.65 g of HCl (molar mass 36.46 g/mol) in 1 L of water, the concentration is 0.1 M.
Click Calculate pH. The calculator applies pH = -log₁₀[H⁺] and shows all related values.
Check the pOH and [OH⁻] fields if you need the full picture for a neutralisation problem.
Strong Acid pH Formula
Because a strong acid dissociates completely, every mole you dissolve releases exactly one mole of H⁺ ions. So the H⁺ concentration equals the acid concentration, and you can plug it straight into the pH formula. A 10-fold decrease in concentration raises the pH by exactly 1.0 unit: 0.1 M HCl gives pH 1.00, and 0.01 M HCl gives pH 2.00.
Worked Examples
Where This Calculation Comes Up
Calculating the pH of a strong acid solution is one of the first things you do in a general chemistry acid-base unit. You need it for titration calculations, where knowing the starting pH of your acid lets you draw the titration curve and identify the equivalence point. Lab reports for acid-base titrations almost always require you to calculate the theoretical pH before you even run the experiment.
Outside of class, strong acid pH calculations are used in water treatment, where operators must know the pH contribution of chemicals added to drinking water. In clinical chemistry, understanding how strong acids affect blood pH is foundational to diagnosing and treating acidosis. Even food scientists use this: calculating the pH of a citric acid or phosphoric acid solution in a beverage formulation depends on knowing whether you are dealing with a complete or partial dissociation, which is where the strong versus weak acid distinction becomes critical.
Frequently Asked Questions
What is a strong acid?
A strong acid is one that completely dissociates in water. Common strong acids include HCl (hydrochloric), HBr, HI, HNO₃ (nitric), HClO₄ (perchloric), and H₂SO₄ (sulfuric, first proton only).
How is pH of a strong acid calculated?
Because strong acids dissociate 100%, the H⁺ concentration equals the acid concentration. pH = −log₁₀[H⁺] = −log₁₀[acid].
What pH does a 0.1 M HCl solution have?
pH = −log(0.1) = 1.00. A 1 M HCl solution has pH = 0. Very high concentrations theoretically give negative pH.
Why can't pH be simply zero for all strong acids?
pH depends on concentration. A 0.001 M HCl has pH 3, while 1 M HCl has pH 0. The formula accounts for this accurately.
What about diprotic strong acids like H₂SO₄?
For the first dissociation (which is complete), treat concentration normally. The second dissociation (pKa ≈ −3) is also considered strong at typical concentrations.