The viscosity of a substance is a measure of how quickly the substance flows when it is subjected to a force such as gravity. The viscosity of a liquid or solution is dependent on: • size of molecules • strength of intermolecular forces of attraction • temperature. It is possible to calculate the mean molecular mass (mean Mr) of a polymer in solution by measuring the viscosity of solutions of the polymer at different concentrations. Measurements related to the viscosity of a solution can be made using a capillary viscometer, shown in the diagram. [Figure X.X] • The apparatus is set up as shown. • The bung is removed and the solution falls through the capillary section. • The time taken for the top of the solution to pass between the two marks at the top (A) and bottom (B) of the reservoir is recorded. • This time taken is related to the viscosity of the solution. A student plans an experiment to calculate the mean Mr of molecules of poly(phenylethene). The student plans to make solutions of different concentrations of poly(phenylethene) dissolved in methylbenzene, C6H5CH3, an organic solvent.