Vapor Pressure Osmometry (VPO) is a reliable technique for experimental measurement of both activity coefficient of solvents and molar weight of solutes. In this work were analyzed the uncertainty sources that affect the measurements on VPO, establishing a general model for uncertainty, and applying this model to the particular case of a VPO prototype designed and constructed in this work as well. In the analysis was included the effect of strong electrolyte solutes of different ratios on the uncertainty of the measurements.

In order to obtain a reliable mathematical model, a simplified thermodynamic model for the activity coefficient was used. Two types of electrolytes (1:1 and 1:2) were used for experimental runs in the diluted and non-diluted concentrations.

The osmometer was designed using thermistors, so, four commercial thermistors of two different types were calibrated in the range of 0-40°C according to ASTM E879 Standard, obtaining an expanded uncertainty of 0.014°C with a confidence level of 95%.

For the uncertainty budget, the relevant sources were identified and quantified. The expected uncertainty of the measurements ranged from 1.03x10^-6 to 1.81x10^-4 for the estimation of the activity coefficient with a confidence level of 95%.