The Measurement of Activity Coefficients in Concentrated Electrolyte Solutions. An Experiment for the Physical Chemistry Laboratory

A physical chemistry experiment is described in which a relatively inexpensive osmometer is used to measure the practical osmotic coefficient of 1:1 strong electrolyte solutions. These data can be used to calculate mean ionic activity coefficients using nonlinear curve fitting and some instructive spreadsheet calculations. Because of data smoothing via the curve fit, the results are a surprisingly satisfactory way for students to measure activity coefficients in the course of a 3-hour laboratory. Student-determined values of the mean ionic activity coefficient are within 4% of literature values for KNO3 solutions from 0.0 to 1.0 m.