Skip to main content
Science & Engineering

pH Calculator

Calculate pH and pOH from [H⁺] or [OH⁻], for strong acids and bases, or for a weak acid with a known Ka.

Your inputs

Acetic acid ≈ 1.8 × 10⁻⁵ at 25 °C.

Results

pH

2.8753

pOH
11.1247
[H⁺]
1.33267E-3 mol/L
Solution is
Acidic
Fraction dissociated
1.333%

≈Estimate based on the stated model and assumptions

Notes for this result

  • Assumes 25 °C (Kw = 1.0 × 10⁻¹⁴) and ideal behaviour (activity = concentration). Concentrated solutions and polyprotic acids need more detailed models.

What this calculator does

pH measures acidity on a logarithmic scale. Different solution types need different calculations.

Formula and method

pH = −log₁₀[H⁺]; pOH = 14 − pH (25 °C) Strong acid: [H⁺] = (C + √(C² + 4Kw)) ÷ 2; weak acid: [H⁺] = (−Ka + √(Ka² + 4KaC)) ÷ 2

Worked example

Calculated by this tool with its default inputs:

Inputs

  • Model: Weak monoprotic acid with Ka
  • Concentration: 0.1 mol/L
  • Ka of the acid: 0.000018

Results

  • pH: 2.8753
  • pOH: 11.1247
  • [H⁺]: 1.33267E-3 mol/L
  • Solution is: Acidic
  • Fraction dissociated: 1.333%

How to read the results

Each pH unit is a tenfold change in hydrogen-ion concentration.

Assumptions and limitations

  • Assumes 25 °C and ideal (dilute) solutions; including water’s autoionisation keeps very dilute acids below pH 7.

These are educational estimates. Electrical, structural and other safety-critical work must comply with local codes and be designed or checked by a qualified professional.

Questions

Why isn’t 10⁻⁸ M HCl at pH 8?

Water itself contributes H⁺; the solution stays slightly acidic (pH ≈ 6.98).

Methodology and verification

Result type
Estimate based on the stated model and assumptions
Formula version
1.0
Automated verification
5 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
Editorial review
Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.

Privacy

Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Inputs are added to a share link only if you choose to include them. Privacy policy