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Henry's Law Calculator

Find dissolved gas concentration with Henry's law.

How it is calculated

Henry's law says the amount of gas dissolved in a liquid is proportional to the gas pressure above it: C = kH × P. kH is the Henry's law solubility constant for that gas, liquid and temperature, and P is the partial pressure of the gas. The result is shown in mol/L to four decimals, so kH must be given in mol/(L·pressure unit) matching the pressure unit you enter (for example mol/(L·atm) with atm). Neither value can be negative. The constant changes with temperature, and the law works best for gases that do not react with the liquid.

Worked example

Take kH = 0.0013 mol/(L·atm), a typical order of magnitude for oxygen in water at room temperature, and a partial pressure of 2 atm. Multiply: 0.0013 × 2 = 0.0026. The dissolved concentration is 0.0026 mol/L. Halving the pressure to 1 atm would halve the concentration to 0.0013 mol/L.

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Common mistakes

  • Mixing pressure units, such as a constant per atm with a pressure in kPa or bar.
  • Using the total pressure instead of the partial pressure of the gas (air is only about a fifth oxygen).
  • Applying a constant found at 25 °C to a cold or hot liquid.
  • Using the inverse-form constant (pressure per concentration) published in some tables; this tool needs the solubility form.
  • Expecting it to work for gases that react strongly with water, like CO₂ at high pressure or ammonia.

About this calculator

Henry's law states that the amount of a gas dissolved in a liquid is directly proportional to its partial pressure above the liquid: C = kH × P. The Henry's law constant kH is specific to each gas-solvent pair and drops as temperature rises, which is why a warm soda goes flat faster than a cold one and why dissolved oxygen in a lake falls as the water warms.

Environmental engineers use it to model how oxygen enters rivers and how volatile pollutants escape from groundwater into air, chemical engineers size gas absorption and stripping columns with it, and it explains decompression sickness in scuba diving, where nitrogen dissolved under pressure comes out of solution too fast on ascent. Beverage and aquarium CO₂ dosing also follow the same relationship.

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