A model of drop with constant adsorption of surfactant arose in drop from soluble condensation nucleus has been considered. Thermodynamic behaviour of chemical potential extremes of substance condensed in drop out of vapor has been shown in this model corresponding to the limiting case of complete saturation adsorption. The validity bounds for the model of drop with constant adsorption have been determined. The threshold value of vapor chemical potential was found. Results of the analytical theory were illustrated by direct calculations of the chemical potential of substance, condensed in drop out of vapor, as a function of the number of molecules of this substance.