Henry’s Law Reference Page
Oh, Henry!

Author

DOFPro group

  • Video links go directly to the YouTube video.
  • JTF (Just the Facts) videos are the streamlined versions: greenscreen LaTeX equations, clean graphics, minimal narrative. Think efficient and to the point. Each has a companion TFS video.
  • TFS (The Full Story) videos include interviews, additional explanation, and equations written on whiteboards. Same math, more context, more personality. Each has a matching JTF version.
  • Info Page links lead to definitions, expanded explanations, and related material—because sometimes you really do need to explain it.
  • Visuals links contain the greenscreen or whiteboard materials used in the video, for those who like to see the scaffolding.
  • Wondering about the titles? See Appendix B: If you have to explain it, it’s no longer funny.
  • Videos marked This is NOT a DOFPro video were not produced by DOFPro but are included because they are relevant to the topic. They are shown in red so no one calls the academic integrity police.

Intro to Henry’s Law

When a gas is in contact with a liquid, some of the gas may dissolve into the liquid phase. The amount that dissolves depends on the gas, the liquid, the temperature, and the pressure.

Henry’s Law describes the equilibrium relationship between the partial pressure of a gas above a liquid and the amount of that gas dissolved in the liquid. This principle is widely used in chemical engineering for problems involving humidification, gas absorption, and gas–liquid equilibrium.

Oh, Henry!

This video introduces Henry’s Law and shows how it is applied to systems containing a single condensible vapor mixed with a noncondensible gas.

Visuals

Examples and Definitions

Definitions

Clapeyron Equation
A thermodynamic equation relating the slope of the vapor-pressure–temperature curve to the enthalpy of vaporization, the absolute temperature, and the molar volumes of the vapor and liquid phases.

\[ \frac{dp^*}{dT} = \frac{\Delta \hat{H}_{v}}{T(\hat{V}_{g} - \hat{V}_l)} \]

Clausius–Clapeyron Equation
An approximate equation relating the vapor pressure of a pure substance to temperature, assuming that the enthalpy of vaporization does not vary with temperature.

\[ \ln p^{*} = -\frac{\Delta \hat{H}_v}{RT} + B \]

Dew-Point Temperature
The temperature at which a condensible vapor begins to condense from a gas–vapor mixture.
Antoine Equation
An empirical equation relating the vapor pressure of a pure substance to temperature.

\[ \log_{10} p^{*} = A - \frac{B}{T + C} \]

Relative Saturation or Relative Humidity
The ratio of the partial pressure of a condensible vapor to its vapor pressure at the same temperature.

\[ s_r \text{ or } h_r = \frac{p_i}{p_i^*(T)} \times 100\% \]

Molal Saturation or Molal Humidity
The ratio of moles of vapor to moles of vapor-free (dry) gas.

\[ s_m \text{ or } h_m = \frac{p_i}{P - p_i} \]

\[ = \frac{\text{moles of vapor}}{\text{moles of vapor-free (dry) gas}} \]

Absolute Saturation or Absolute Humidity
The ratio of the mass of vapor to the mass of vapor-free (dry) gas.

\[ s_a \text{ or } h_a = \frac{p_i M_i}{(P - p_i) M_{dry}} \]

\[ = \frac{\text{mass of vapor}}{\text{mass of vapor-free (dry) gas}} \]

Percentage Saturation or Percentage Humidity
The ratio of the molal saturation to the molal saturation at equilibrium.

\[ s_p \text{ or } h_p = 100\% \times \frac{s_m}{s_m^*} \]

\[ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 100\% \times \frac{\frac{p_i}{P-p_i}}{\frac{p_i^*}{P-p_i^*}} \]