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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 11 Issue 3 (2006) pp 172-173

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

Frazier Nyasulu,* Marianne Cavelti, and John Macklin

Department of Chemistry, University of Washington,Box 351700, Seattle, WA 98195,
Received September 28, 2005. Accepted December 4, 2005

Published online: 1 May 2006

Abstract. 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.

Key Words: Laboratories and Demonstrations; general chemistry; ideal gas law constant; molar mass

(*) Corresponding author. (E-mail:

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Issue date: June 1, 2006

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