An Inexpensive Microscale Method for Measuring Vapor Pressure, Associated Thermodynamic Variables, and Molecular Weight
Jason C. DeMuro, Hovanes Margarian, Artavan Mkhikian, Kwang Hi No, and Andrew R. Peterson
U. S. Grant High School, 13000 Oxnard Street, Van Nuys, CA 91401
Existing methods for measuring vapor pressure are too expensive or not quantitative enough for chemistry classes in secondary schools. Our method measures the vapor pressure inside a bubble trapped in a graduated microtube made from a disposable 1-mL glass pipet. Vapor pressures of water, methanol, and ethanol are measured over temperature ranges of 4-90 °C. The enthalpy and entropy of vaporization and boiling points, calculated using the Clausius-Clapeyron equation, agree well with published values. The vapor pressures of aqueous solutions of ethanol and methanol plotted against mole fractions of water give positive deviations from Raoult's law, but concentrations were identified from which molecular weights of the alcohols could be calculated. These molecular weights are not significantly different from published values. Sources of error in the method are analyzed. A procedure for use in secondary schools is outlined.
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Citation
DeMuro, Jason C.; Margarian, Hovanes; Mkhikian, Artavan; No, Kwang Hi; Peterson, Andrew R. J. Chem. Educ.1999 76 1113.
Keywords
Introductory / High School Chemistry; Laboratory Instruction; Aqueous Solution Chemistry; Gases; Microscale
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