-,
GEORGE L.
GILBERT
Denison University Granville. Ohio 43023
A Simple Apparatus Designed to Measure Vapor Pressures and Demonstrate the Principles of R~OUI~'S L ~ W s w w m wv: E. Koubek U S Naval
Academy Annapolis. MD 21402 CHECKED BY: Donald R. Paulson California State University LOS Angeles. CA An apparatus which we find very helpful in introducing the concepts of vapor pressure and Raoult's Law is shown in the fieure. First, one sets the mercury levels on a fixed mark on thk left-hand side of the apparatus with the sample hulh open to the atmosphere' (This is accomplished moving the right-hand column of mercury UP or down and then locking it in place by tighteninga butterfly nut in the hack). Next, the ruhher septum is set in place and about 1ml of cyclohexane is introduced into the hulh via a hypodermic syringe. After a few minutes the right-hand mercury column is again moved
until the left-hand mercury level has returned to the previously established reference point. This assures that the original air in the hulb is a t the same volume and that the increase in nressure within the svstem is due to the vaoor . .nressure of henzene. Then, from the difference in mercury levels, one can actually measure the vapor pressure of henzene. Next, we introduce into the hulb another sample, this time 1 ml of xylene, hut before we do, we ask the class to predict what will happen. They always predict that the pressure will go up even more and are amazed to see it go down-setting the stage for a discussion of Raoult's Law.' Although the above principles may he illustrated in other ways, we like this particular apparatus because the liquids never come in contact with the mercury. Thus, the apparatus may he used again and again without cleaning up a column of mercury each time.
' In this discussion we find it is best not to make too much of the actual measured value for the solution as it may not be as expected based on a simple Raouk's Law calculation. This is especially true when only small quantities of liquid are injected into the apparatus. See. Koubek, E., and Elert, M., J. CHEM.EOUC., 59,357 (19821.
Alcohol-Fueled Missile S U B M IBY: ~O
Joseph W. Bozzelll New Jersey Institute of Technology Newark, NJ 07102 CHECKED BY:
Richard F. Jones Sinclair Community College Dayton, OH 45459
I
This lecture demonstration is a variation on orevious ones in which either a glass vessel or a metal container was utilized to contain the comhustion of a volatile gaseous mixture. These reactions include the ignition of hydroken or volatileorganics with air or oxygen. The use ofglass hottlw' or containers-'.., allows the possibility of hreakage and flying glass pieces, while metal containers limit the visihility of the experiment and reactions within. This demonstration uses a clear polypropylene or polyethylene container to illustrate the energy available from alcohol comhustion by propelling a rubber stopper and length of plastic tuhing a distance of approximately 20-30 m with a noticeable, hut not deafening, pop. The equipment consists of a one-holed ruhher stopper containing a 20-25 cm length of plastic tubing (with a short piece of soft tubing on each tip), a 400-600 ml clear plastic hottle, a Tesla coil, and a spark plug or two pieces of insulated electrical wire. The spark plug is threaded and glued into the wall of the vessel. or one end of each wire is inserted through a small opening so that iw insulation makes a seal at the w h 1 of the bottle. Additionallv, the spark nlua, if it ib used. is tied to the container with a st& or blast& wire. The plug should be aimed in a safe direction in case it does become a missile. T h e assembly is shown in the figure. To perform the demonstration 4-6 ml of alcohol (methanol or ethanol) is placed in the plastic bottle and the stoppertuhing assembly is inserted firmly. The base of the spark plug or end of one wire is attached to a suitable ground (electrical Volume 60 Number 12 December 1983
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