Determination of the Universal Gas Law Constant ( R ) and Molar Mass of a Low-Boiling Liquid Using Gas-Pressure Sensors

In this experiment, 0.50-mL aliquots of pentane are incrementally injected into a large evacuated filtering flask that is maintained at constant temperature (100 °C). The pressure in the flask is measured after each addition using a gas-pressure sensor. This procedure allows multiple determinations of the gas constant R to be performed quickly and accurately, with more than 85% of the students reporting a value of R that is within 5% of the literature value. Alternatively, when the injected liquid is treated as an unknown, this setup is used to determine molar mass. Pentane was selected for the determination of R; ethanol, acetone, 2-propanol, and 2-butanone were selected for the determination of molar mass. Ethanol and 2-butanol are differentiated from the other compounds at the 99% confidence level; however, acetone and 2-propanol are not. The inclusion of acetone and 2-propanol illustrates the need for greater accuracy and precision when distinguishing between similar compounds.