Chemistry

pH Scale Calculator

Enter a pH, pOH, hydrogen ion or hydroxide ion concentration, or the molarity of a strong acid or base. You get all four values, the acid/base classification and a marker on the pH scale with everyday substances for comparison.

Scientific notation: 1e-7, 1x10^-7 or 10^-7. *Treats both protons of sulfuric acid as fully released (an upper estimate).

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Working
Common substanceTypical pH

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Why use a pH calculator

Check homework fast

Convert between all four quantities in one step and see the formula used.

Avoid the dilute-acid trap

Exact water correction for very weak concentrations.

Visualise acidity

Place a result next to lemon juice, blood or bleach.

How to calculate pH

  1. Choose what you knowpH, pOH, [H+], [OH-] or a strong acid or base molarity.
  2. Enter the valueDecimals or scientific notation like 2.5x10^-4 both work.
  3. Read the resultsSee every related value, the classification and the scale marker.

Understanding the pH scale

The pH scale is logarithmic: each whole step is a tenfold change in hydrogen ion concentration. A solution at pH 3 has ten times more H+ than one at pH 4 and a thousand times more than one at pH 6.

At 25 °C pure water has [H+] = [OH-] = 1.0 x 10^-7 mol/L, so neutral is pH 7. Warmer water ionises more, so its neutral pH is lower (about 6.6 at 50 °C) even though it is not acidic.

Acidic

pH below neutral: [H+] is greater than [OH-].

Neutral

pH = pKw / 2: equal H+ and OH-.

Basic (alkaline)

pH above neutral: [OH-] is greater than [H+].

pH calculator FAQ

How do you calculate pH from [H+]?

pH = -log10[H+], with the concentration in mol/L. For example [H+] = 1 x 10^-3 mol/L gives pH 3.

How do I get [H+] from pH?

[H+] = 10^-pH. A pH of 5.5 means [H+] = 10^-5.5 = 3.16 x 10^-6 mol/L.

What is the relationship between pH and pOH?

pH + pOH = pKw. At 25 °C pKw is 14.00, so pOH = 14 - pH. At other temperatures pKw changes, which is why the neutral point is not always exactly 7.

Why is the pH of a very dilute acid not above 7?

Water itself supplies about 10^-7 mol/L of H+. The strong acid mode solves [H+] = (C + sqrt(C^2 + 4Kw)) / 2, so even a 10^-9 M acid stays just below pH 7 instead of the incorrect 9.

How do I enter numbers in scientific notation?

Type 1e-7, 1x10^-7, 1*10^-7 or 10^-7. All of these mean 0.0000001.

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