Online Chemistry Labs | Learning Objectives


This experiment allows students to determine the molecular weight of three compounds by measuring the osmotic pressure of the aqueous solutions formed when a measured mass of each compound is dissolved in a known volume of water. A laboratory osmometer is used for the measurements. The dissolved materials include an inorganic ionic compound of stated molecular weight, sucrose, and a protein of unknown molecular weight. The standard equation for the dependence of osmotic pressure on Molarity is used, and the van't Hoff factor for each compound is given. The approximate molecular weight of the unknown can be determined.  Limitations of the methodology are discussed.

 Osmotic Pressure

Learning Objectives:

  1. Become familiar with the operation of a laboratory osmometer.
  2. Learn how to calculate the Molarity of the solution from the measured osmotic pressure, given the van't Hoff factor.
  3. Be able to calculate the approximate molecular weight once the Molarity is known.
  4. Be able to answer questions about the limitations of the method.