Henry’s Law Converter
Convert a Henry’s law constant between mol/(L·atm), atm·L/mol, mol/(m³·Pa), mole-fraction forms, atm·m³/mol and cm³/cm³. Enter the value, pick the From and To units, and add a temperature when cm³/cm³ is involved.
Convert Henry’s Constant Between Units
Henry’s Law Formula
Where:
- \(c\) — dissolved-gas concentration (mol/L).
- \(p\) — partial pressure of the gas above the liquid (atm; Pa forms use 1 atm = 101,325 Pa).
- \(k_{c/p}\) — Henry’s law constant in mol/(L·atm), the base definition the calculator converts everything through.
- \(k_{p/c}\) — the reciprocal constant in atm·L/mol: \(k_{p/c}=1/k_{c/p}\).
- \(k_{p/x}\) — pressure per dissolved mole fraction, in atm/mol fraction; the Pa/mol fraction value is 101,325 times the atm/mol fraction value.
- \(C_w\) — molar concentration of the liquid; the calculator uses \(C_w \approx 55.5\) mol/L, the value for water, with \(x \approx c/C_w\) for dilute solutions.
- \(k_{\mathrm{SI}}\) — the same constant in mol/(m³·Pa), obtained from 1 m³ = 1,000 L and 1 atm = 101,325 Pa.
- \(k_{cc}\) — dimensionless solubility in cm³/cm³ (volume of gas, measured at the gas-phase conditions, per volume of liquid).
- \(R = 8.2057\times10^{-5}\) atm·m³/(mol·K) — the gas constant, and \(T\) — absolute temperature in kelvin.
Henry’s law itself is the first equation, \(c = k_{c/p}p\): the dissolved concentration grows in proportion to the gas’s partial pressure. The remaining relations convert one published definition of the constant into another, and the calculator applies them in two steps: first to mol/(L·atm), then to the requested unit. Temperature enters only the cm³/cm³ conversion.
Henry’s Law – Calculation Example
Example: keep the default inputs — a Henry’s constant value of 0.034 (a typical value for CO₂ in water at 25 °C), From unit mol/(L·atm), To unit atm/mol fraction. For a dilute aqueous solution, \(x \approx c/55.5\), so:
\[ k_{p/x} = \frac{1}{k_{c/p}\,C_w} = \frac{55.5}{0.034} = 1632.353 \ \text{atm/mol fraction} \]
Result: 1632.353 atm/mol fraction. As a cross-check, the Pa/mol fraction form is 101,325 times larger, about \(1.654\times10^{8}\) Pa/mol fraction.
How to use the calculator
- Enter the number in Henry’s constant value. It must be greater than zero, because several definitions are reciprocals of one another.
- Select From unit to match the definition under which your value was reported — concentration per pressure, pressure per concentration, pressure per mole fraction, or dimensionless. Do not match by name alone; check the source’s units.
- Select the To unit you need.
- If either unit is cm³/cm³, the Temperature field appears; enter the absolute temperature in kelvin (the default, 298.15 K, is 25 °C). This field is ignored for every other pair of units.
- Read the result under Converted Henry’s Constant.
What a Henry’s law constant tells you
Henry’s law states that, at equilibrium and within a range where the relationship stays linear, the amount of a gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. Written as \(c = k_{c/p}p\), a larger constant means the gas dissolves more readily: oxygen dissolves in water more readily than nitrogen in the sense that its \(k_{c/p}\) is larger. Written the other way, \(p = k_{p/c}c\), a larger constant means a higher pressure is needed to hold a given concentration in solution — the same physical information, expressed as a reciprocal number.
The constant is reported in many different forms because different fields measure different quantities. Environmental and aquatic-chemistry tables often use mol/(L·atm); engineering references often use atm·m³/mol; geochemistry and physical-chemistry compilations frequently use pressure per mole fraction, atm/mol fraction or Pa/mol fraction; gas-solubility tables may quote a dimensionless ratio in cm³ of gas per cm³ of liquid. The numbers differ by orders of magnitude even for the same gas at the same temperature, so a converter is essential when comparing values from different sources.
The mole-fraction forms used here assume a dilute solution in water, where the mole fraction of dissolved gas is \(x \approx c/55.5\). The default example above illustrates the scale: 0.034 mol/(L·atm) for CO₂ becomes about 1,632 atm/mol fraction. If you also need to work with the pressure side of the equation in other units, the Pressure converter handles atm, Pa, bar and related units. For gas-phase properties that depend on conditions, the Air Density calculator can supply the density that appears alongside Henry’s law in mass-transfer problems.
The table below shows how all seven supported units relate to one another, using the default value \(k_{c/p} = 0.034\) mol/(L·atm) at 298.15 K:
| Unit | Definition | Value for 0.034 mol/(L·atm) |
|---|---|---|
| mol/(L·atm) | concentration per pressure (base form) | 0.034 |
| atm·L/mol | pressure per concentration, \(1/k_{c/p}\) | 29.41176 |
| mol/(m³·Pa) | SI concentration per pressure | 0.000335553 |
| atm/mol fraction | pressure per dissolved mole fraction | 1632.353 |
| Pa/mol fraction | pressure per dissolved mole fraction (SI) | \(1.654\times10^{8}\) |
| atm·m³/mol | pressure per concentration, volume in m³ | 0.02941176 |
| cm³/cm³ | gas volume per liquid volume, at 298.15 K | 1.202 |
Assumptions and limitations
- Dilute aqueous solution. The mole-fraction and cm³/cm³ conversions use \(C_w = 55.5\) mol/L, the molar concentration of pure water, and \(x \approx c/C_w\). This is only valid for sparingly soluble gases in water. For other solvents, or concentrated solutions, use the solvent’s actual molar concentration instead.
- Reciprocal definitions. mol/(L·atm) and atm·L/mol are reciprocals, not equivalent units: 0.034 mol/(L·atm) is 29.412 atm·L/mol, not 0.034. Likewise atm·m³/mol is the m³-based reciprocal, smaller by a factor of 1,000.
- Temperature dependence. Henry’s law constants change strongly with temperature (gases are generally less soluble at higher temperature). The temperature field affects only the cm³/cm³ result; a constant quoted at one temperature is not valid at another.
- Ideal-gas behavior. The cm³/cm³ form assumes the dissolved-gas volume follows \(pV = nRT\) with the gas constant in atm·m³/(mol·K), which is accurate at low pressures but degrades at high pressure.
- Linearity range. Henry’s law itself holds only at low partial pressures and low concentrations. At high pressures, chemical reactions of the gas with the liquid (for example CO₂ forming carbonic acid) can make the simple proportionality invalid.
- Check the source convention. Published compilations use overlapping symbols (\(k_H\), \(H\), \(K_H\)) for different definitions. Always confirm the units attached to a value before entering it, and verify a converted value against the source’s stated convention when accuracy matters.
Frequently Asked Questions
How do I calculate dissolved-gas concentration using Henry’s law?
Multiply the gas’s partial pressure by its concentration-per-pressure constant: \(c=k_{c/p}p\). For example, with \(k_{c/p}=0.034\) mol/(L·atm) for CO₂ in water at 25 °C and a partial pressure of 0.50 atm, \(c=0.017\) mol/L. Use a constant measured for the same gas, solvent, and temperature, and stay within the low-pressure range where the proportionality holds.
Is there a normal or good value for a Henry’s law constant?
No single value is normal or good. A Henry’s law constant describes one gas–liquid pair at one temperature, and published sources report it in different definitions. Before comparing values, match the gas, solvent, temperature, and unit form. A larger concentration-per-pressure constant means greater solubility; the interpretation reverses for pressure-per-concentration reciprocals, where a larger number means lower solubility.
How do I convert between mol/(L·atm) and atm·L/mol by hand?
Take the reciprocal, because these two definitions are inverses of each other: \(k_{p/c}=1/k_{c/p}\). For example, 0.034 mol/(L·atm) becomes \(1/0.034\approx29.412\) atm·L/mol — not 0.034. The related atm·m³/mol form uses cubic meters, so its value is the atm·L/mol number divided by 1,000: 29.412 atm·L/mol is 0.029412 atm·m³/mol.
Should I use total pressure or partial pressure in Henry’s law?
Use the dissolving gas’s partial pressure, not the total pressure of the mixture. For an approximately ideal gas mixture, multiply total pressure by that gas’s mole fraction in the gas phase: \(p_i=y_iP\). A gas making up 20% of a mixture at 2 atm has a partial pressure of about 0.4 atm. Do not confuse the gas-phase mole fraction with the dissolved mole fraction used in pressure-per-mole-fraction constants.
Can I use a Henry’s law constant measured at a different temperature?
Not directly. Gas solubility changes strongly with temperature — gases are generally less soluble in warmer water — so a constant quoted at one temperature is not valid at another. Some compilations give empirical temperature functions or van ’t Hoff–type parameters, but you must apply them with the same definition of the constant the source used. For accurate work, find a measured value at your target temperature.
What does the cm³/cm³ form mean, and why does converting it require temperature?
cm³/cm³ is a dimensionless solubility: the volume of dissolved gas, measured at gas-phase conditions via the ideal-gas law \(pV=nRT\), per volume of liquid. Because the gas volume depends on absolute temperature, the conversion to or from any other unit needs the temperature in kelvin — that is why the Temperature field appears only when cm³/cm³ is involved. The default 298.15 K corresponds to 25 °C.
How do I convert mol/(L·atm) to the SI form mol/(m³·Pa) by hand?
Multiply the mol/(L·atm) value by 1000/101,325, combining the volume change (1 m³ = 1,000 L) and the pressure change (1 atm = 101,325 Pa) in one step. For example, \(0.034\times1000/101325\approx3.36\times10^{-4}\) mol/(m³·Pa). Convert the volume and pressure parts separately if that helps you keep the factors straight, but check the direction of each one.
Your need to include a conversion from atm-m3/mole to cm3/cm3. That is what is most commonly used in environmental calculations.
We added a conversion from atm-m3/mole to cm3/cm3