Water Activity - Formulae

Formulae

Definition of a_w:

where p is the vapor pressure of water in the substance, and p₀ is the vapor pressure of pure water at the same temperature.

Alternate definition:

where l_w is the activity coefficient of water and x_w is the mole fraction of water in the aqueous fraction.

Relative humidity:

The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH).

Estimated mold-free shelf life in days at 21° C: