Engineering · Chemical Engineering · Separation Processes
Henry's Law Calculator
Calculate the concentration of a dissolved gas in a liquid using Henry's Law, relating partial pressure to solubility at equilibrium.
Calculator
Formula
Where p is the partial pressure of the gas above the liquid (in Pa or atm), K_H is the Henry's Law constant (in Pa·m³/mol or atm·L/mol, depending on convention), and c is the molar concentration of the dissolved gas in the liquid (in mol/m³ or mol/L). The relationship is linear at dilute concentrations and constant temperature. Rearranged: c = p / K_H to find concentration, or K_H = p / c to determine the constant experimentally.
Source: IUPAC Recommendations on Henry's Law Constants, Pure and Applied Chemistry, Vol. 82, No. 5 (2010); Sander, R. (2015), Atmos. Chem. Phys., 15, 4399–4981.
How it works
Henry's Law states that at constant temperature, the concentration of a dissolved gas in a liquid is directly proportional to the partial pressure of that gas above the liquid surface. This linear relationship holds well for dilute solutions and sparingly soluble gases — conditions commonly encountered in industrial absorption columns, aeration tanks, and environmental fate modeling. The law breaks down at high pressures or high concentrations, where non-ideal behavior becomes significant.
The governing equation is p = K_H × c, where p is the partial pressure of the gas (atm), K_H is the Henry's Law constant (atm·L/mol), and c is the molar concentration of dissolved gas (mol/L). Note that Henry's Law constants can be expressed in multiple conventions — some sources use the inverse form (c = K_H × p), so always confirm the convention being used. This calculator uses the volatility convention: higher K_H indicates lower solubility and greater tendency for the gas to remain in the vapor phase.
Practical applications include calculating dissolved oxygen in natural water bodies, sizing scrubbers and absorption towers for air pollution control, modeling CO₂ dissolution in oceans and carbonated beverages, designing vacuum degassing systems, and assessing the fate of volatile organic compounds (VOCs) in groundwater. Environmental engineers routinely use Henry's Law to determine whether a contaminant will partition into the atmosphere or remain in water, which drives remediation strategy selection.
Worked example
Example: Dissolved Oxygen in Water at 25°C
Atmospheric air contains approximately 21% oxygen, giving an oxygen partial pressure of about 0.21 atm at standard atmospheric pressure (1 atm total).
The Henry's Law constant for O₂ in water at 25°C is approximately K_H = 769.23 atm·L/mol (volatility convention).
Using Henry's Law to find the dissolved oxygen concentration:
c = p / K_H = 0.21 atm / 769.23 atm·L/mol = 0.000273 mol/L
Converting to mg/L for comparison with water quality standards:
c = 0.000273 mol/L × 32 g/mol × 1000 mg/g ≈ 8.74 mg/L
This result aligns well with the known dissolved oxygen saturation of water at 25°C (~8.2–8.7 mg/L), confirming the calculation. This value is critical for assessing aquatic ecosystem health — most fish require at least 5 mg/L of dissolved oxygen to survive.
If we instead needed to find the partial pressure that would achieve a target concentration of 0.0004 mol/L (hyperbaric oxygen therapy scenario):
p = K_H × c = 769.23 × 0.0004 = 0.308 atm
This means an oxygen partial pressure of about 0.308 atm — achievable in an oxygen-enriched or pressurized environment — would be needed to dissolve that concentration.
Limitations & notes
Henry's Law is a limiting law valid only under specific conditions. It applies strictly to dilute solutions where gas-liquid interactions are minimal and at a fixed temperature — K_H values change significantly with temperature, typically increasing (higher volatility) as temperature rises, so always use the constant appropriate for the system temperature. The law assumes ideal gas behavior in the vapor phase and infinite dilution in the liquid phase; at high pressures or high dissolved gas concentrations, corrections using activity coefficients (Raoult's Law, fugacity models) are required. Henry's Law does not apply to gases that react with the solvent — for example, CO₂ partially converts to carbonic acid in water, and HCl fully dissociates, making the apparent Henry's constant strongly pH-dependent. Multiple dissolved gases are treated independently; interactions between solutes (salting-out, salting-in effects) are not captured. Additionally, this calculator assumes steady-state equilibrium — in dynamic systems with mass transfer resistance, additional kinetic models (two-film theory, penetration theory) must be layered on top of the equilibrium calculation.
Frequently asked questions
What units does Henry's Law constant use and why does it vary between sources?
Henry's Law constants appear in multiple unit conventions because the law can be written with concentration on either side. The volatility form (p = K_H × c) uses atm·L/mol or Pa·m³/mol, while the solubility form (c = K_H × p) uses mol/L/atm. Always identify which convention a reference uses before plugging in values. This calculator uses the volatility convention where larger K_H means less soluble.
How does temperature affect Henry's Law constants?
Henry's Law constants increase with temperature for most gases, meaning gases become less soluble as water warms — a well-known cause of reduced dissolved oxygen in warm summer rivers. The van't Hoff equation can be used to adjust K_H for temperature: ln(K_H(T2)/K_H(T1)) = (ΔH_sol/R)(1/T1 − 1/T2), where ΔH_sol is the enthalpy of dissolution. Always use the tabulated K_H at the actual system temperature for accurate results.
Can I use Henry's Law for CO₂ dissolved in water?
Yes, but with important caveats. CO₂ physically dissolves according to Henry's Law, but it also chemically reacts with water to form carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻) depending on pH. The Henry's Law constant only describes the physical dissolution step. In systems where chemical reactions occur, the apparent or effective solubility is much higher than the purely physical Henry's constant predicts, and speciation modeling (e.g., with pH equilibrium calculations) is required.
What is the difference between Henry's Law and Raoult's Law?
Both laws describe vapor-liquid equilibrium, but they apply to different components. Raoult's Law applies to the solvent (major component) and relates its vapor pressure to its mole fraction using the pure-component saturation pressure. Henry's Law applies to the solute (minor component, typically a dissolved gas) and is valid in the limit of infinite dilution. At dilute conditions, Raoult's Law applies to the solvent while Henry's Law applies to the dissolved gas simultaneously.
How is Henry's Law used in environmental engineering?
Environmental engineers use Henry's Law to assess whether pollutants will volatilize from water into the atmosphere or remain dissolved — a key factor in risk assessment and remediation design. Compounds with high K_H values (like trichloroethylene or benzene) tend to volatilize and may be removed by air stripping, while compounds with low K_H (like phenol or many pesticides) stay in the aqueous phase and require other treatment methods such as activated carbon adsorption or biological treatment.
Last updated: 2025-01-15 · Formula verified against primary sources.