Henry's Law formula
| Symbol | Quantity | SI unit | Meaning |
|---|---|---|---|
| C | Dissolved gas concentration | mol/m³ | Equilibrium concentration of dissolved gas |
| k_H | Henry's law constant | mol/(m³·Pa) | Henry's law solubility constant (temperature dependent) |
| P | Partial pressure of gas | Pa | Partial pressure of the gas above the liquid |
What is Henry's Law?
Henry's Law: At constant temperature, the amount of a gas that dissolves in a liquid is proportional to its partial pressure above the liquid. — William Henry, 1803
Henry's law describes gas–liquid equilibrium for dilute solutions. Gas molecules continually dissolve into and escape from the liquid, and at equilibrium the dissolved concentration scales with the partial pressure. This calculator uses the solubility form, with kH in mol/(L·atm).
Henry's law explains carbonated drinks fizzing when opened, decompression sickness in divers, oxygen availability for aquatic life, and gas transfer across the lungs. Gas then moves through the liquid by diffusion, so Henry's law sets the boundary concentration for Fick's law.
Henry constants come in several conventions (C/P, P/C, dimensionless). Make sure yours is "concentration per pressure" before entering it. Approximate kH values in water at 25 °C, in mol/(L·atm): O₂ 0.0013, N₂ 0.00065, CO₂ 0.034.
How to use this calculator
- Choose the quantity you want to find in the Calculate menu. It is highlighted as the result field.
- Enter the known values. Scientific notation such as
1.6e-11is accepted. - Select a unit for each value. Units are converted to SI automatically.
- The answer updates as you type. Open Show calculation steps to see the working, or use Copy result to share a link with your inputs.
Worked example
How much oxygen dissolves in water at 25 °C from air (PO₂ = 0.21 atm)?
- C = 0.0013 × 0.21 = 2.73 × 10⁻⁴ mol/L = 0.273 mmol/L.
- × 32 g/mol ≈ 8.7 mg/L, close to the measured saturation of ≈ 8.3 mg/L.
Frequently asked questions
Does Henry's law depend on temperature?
Yes. Most gases become less soluble as temperature rises, which is why warm water holds less oxygen. Always use k_H at your actual temperature.
When does Henry's law not apply?
It fails at high pressure, at high dissolved concentration, and for gases that react with the solvent (e.g. NH₃, or CO₂ forming carbonic acid, which raises its apparent solubility).