Determination of the average molecular weight of petroleum cuts by vapor pressure depression

The authors have constructed a static apparatus for the determination of the average molecular weight of petroleum cuts, by the vapor pressure depression method, also called tonometry. The solvent vapor pressure decrease, which appears when a compound is added in small quantities, is approximately proportional to the solute mole fraction. The average molecular weight of real petroleum cuts is given with a relative error of less than 2%. This error mainly results from the estimation method of the activity coefficients in the solution corresponding to the pseudobinary system “petroleum cut + solvent”.