where "x" is the pressure divided by the vapor pressure for the adsorbate at that temperature (usually denoted formula_25), "v" is the STP volume of adsorbed adsorbate, "vmon" is the STP volume of the amount of adsorbate required to form a monolayer, and "c" is the equilibrium constant "K" we used in Langmuir isotherm multiplied by the vapor pressure of the adsorbate. The key assumption used in deriving the BET equation that the successive heats of adsorption for all layers except the first are equal to the heat of condensation of the adsorbate.