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SUNY-Collland
A Simple and Convenient Microscale Procedure for Investigation of Charles' Law Donald M. Snyder Eastem Michigan University Ypsilanti, MI 48197 An experiment dealing with one or more of the gas laws is a very common part of most undergraduate general chemistry laboratory programs. A study of the volume versus temperature relationship at constant pressure (Charles'~awjnot only exposesstudents to one ofthe fundamental principles ofgas behavior, but also allows them to make an kxperimentai determination of absolute zero. This is particularly useful because students generally find absolute zero to be an abstract and difficult concept. Using their own data to calculate a value for it is a pedagogically powerful demonstration of the value of reinforcing classroom learning with laboratory work. This article describes a method for accurately controlling the temperature and measuring the volume of a gas sample with inexpensive, readily available equipment. While the experimental procedure is quite simple, the accuracy of the results can be quite good with careful measurements. The basic principle of the experiment is to trap a bubble of air within an inverted U-shaped piece of glass tubing submerged in a large test tube of mineral oil, as shown in Figure 1.This is most conveniently done by using a disposable pipet or medicine dropper to fill the tube approximately half way with oil and then inverting it into the oil bath. The oil bath is heated slowly with a Bunsen burner (recording the temperature with a thermometer) and the volume change of the expanding bubble is determined by measuring the level of oil in the U-tube with an external centimeter scale. Because the volume of oil displaced by the expanding gas is very small with respect to the total volume of the oil bath, the slight change in hydrostatic pressure is negligible and a constant pressure can be assumed. The contribution of the vapor pressure of silicone or mineral oil to the pressure of the trapped air also is negligible over the temperature range used, (which can be extended to higher temperatures than if water were used in the bath.) The gas bubble level can be related to the actual volume using a prepared calibration plot, so that a graph of volume versus "C can be plotted. To . prepare the equipment, a . . . Gas expansion piece of standard glass tubing Figure tube in oil bath with external w t h an ID of 4-5 mm is cut to a scale for measurement of length of 18-20 cm. After firebubble level. polishing the ends, the center is A98
Journal of Chemical Education
Dinmee Irom tube bollom (cm)
Figure 2. Graphical plot of expansion tube calibration data.
Temperature (C)
Figure 3. Graphical plot of bubble volume versus temperature extrapolated to zero volume. softened in the Bunsen burner flame and the tube is bent carefully into a hairpin shape with the arms parallel and close enough to fit inside a large test tube. As long as the tube remains open all the way around the radius of the curve, it does not matter if the ID is uniform since this will be accounted for in the volume calibration. After cooling, this tube is calibrated by adding water to fill the arms at l-cm increments, measuring from the bottom of the curve, and weighing aRer each addition (i.e., 1.00 cm = 0.21 g, Experimental Results for Bubble Level versus Temperature TF)
Level (cm)
Volume (mL)
26 45 60 75 80 95 110
5.40 5.70 6.M) 6.30 6.35 6.60 6.85
1.24 1.31 1.38 1.45 1.46 1.52 1.58
2.00 centimeters = 0.45 g, etc.) The mass of water in grams, and hence the volume in milliliters, can be plotted versus measured distance in centimeters, as shown in Figure 2. Experimental data for a Charles' Law determination using this procedure are shown in the table, where the measured gas bubble level at each temperature was converted to an actual volume using the calibration plot of Figure 2. Linear regression analysis of the data points extrapolated to zero volume (Fig. 3) yields a value for absolute zero of 276 'C, in excellent agreement with the accepted value. In this particular run, data were taken only while the oil was heating but additional data points also could be collected while the system is cooling back to ambient conditions. One of the most useful aspects of this procedure is that once the bubble tube is prepared and calibrated, it can be used repeatedly to collect volume/temperature data without disturbing the equipment setup.
Microscale Ninhydrin Test Applied to Solid-Phase Peptide Synthesis Lluis Vilaseca and Eduard Bardaji University of Girona PI. Hospital 6 1707, Girona, Spain
The reaction of ninhydrins has been used extensively for the detection of free primary amino m u ~ sIts . use as a test during the sGihes;s of peptides ria solid-phase methodolow ( 1 , 2 ) makes it &I efficient tool f~f-~e@tide chemists to ensure complete formation of each peptide bond (3).It is also a useful tool for assessing reaction yields when carrying out difficult couplings. Here we report a microscale modification of the ninhydrin test, which allows the use of very small portions of peptidyl-resins and reagents, employing melting point tubes. The general procedure is illustrated in the figure. First, a small portion of washed peptidyl-resin is taken with one end of an open melting point tube. One drop of ninhy-
Volume 72 Number 5 May 1995
A99
