An Inexpensive, Easily Constructed Vapor Pressure Apparatus In many undergraduate physical chemistry laboratory courses, students are required to measure accurately the vapor pressure of pure liquids at various temperatures. The data are then used to calculate the enthalpy of vaporization (AH,,) via the Clausius-Clapyron equation. Experimentally, these procedures are often carried out using the Tohey modification' of the classic Ramsay-Youngz apparatus. However convenient, this apparatus is not universally available to all students. A simple and inexpensive way to construct a vapor pressure apparatus using equipment readily available in most undergraduate laboratories is herein descrihed. The figure illustrates the construction and suggested dimensions. Critical portions of the apparatus consist of three basic parts a) a reservoir (250-ml round-bottomed flask) in which liquid is vaporized, h) a Claisen adapter fitted with a thermometer, which behaves as a column in which the vapor is in equilibrium with condensing liquid, and c) a water-cooled condenser with a distillation head adapter, to prevent vapor from escaping into the remainder of the apparatus. AU glass connections are made with standard-taper ground-glass joints. The reservoir is normally half filled with the liquid of interest and heated by means of a heating mantle under Variac control. Bumping of the liquid is prevented by addition of boiling chips. The thermometer can he any precision type graduated to 0.1T. Range is dependent upon the "normal" boiling point of the sample.3 The distillation adapter is attached to the remainder of the system by rubber or Tygon vacuum tubing and glass T-joints. A 4-L round-bottomed flask, properly shielded to minimize eaplosion/implasion hazard, is used as a ballast tank and attached to a vacuum trap. The manometer is a conventional, closed-end, meter stick manometer (Hg). A vacuum pump is employed to provide reduced pressures. The air-inlet system consists of a two-way stopcocklocated just prior to the ballast tank (see figure).
As descrihed, the apparntus prorides a usable pressure range from 10 to 76CI mm HKuith an accuracy 01 + I mm Ha. Experimentally determined vapor pressure W N e s for water, henzene, m~thanol, acetone, and several othrr common organic 80lvents agree well with value~quotedin the literaturr ( < l ? error).The system mbe easily adapted to obmin meauremrnrs slightly above 760 mm Hg by use of compressed air.'
' Tohey, S. W., J. CHEM.EDUC.,35,352 (1958)
Ramsay, W., and Young, S., J. Chem. Sac., 41,42 (1885). n shown in the figure. Thermometer bulb must he located just below Claisen adapter g ~ as Suitable clamps for all connections must he provided when operating above 760 mm Hg.
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Peter M. Schaber Canisius College 2001 Mam Street Buffalo.NY 14208
Volume 62
Number 4
April 1985
345
