Ronald S. Strange and Frank 1.Lang Fairleigh Dickinson University, Florham-Madison Campus, Madison, N J 07940 There have been several Charles's law experiments published that are simple but suffer from lack of accuracy and precision. Some of these, such as experiments involving balloons ( 1 3 ) .water disulacement ( 4 , 5 ) ,or earn (61,are better for qualitative demonstrations than for theaccurate determination of absolute zero. Those involving liquid mercury (7)have environmental drawbacks. One experiment (a), using a syringe, suffers from nonuniformity of gas temperature and from the presence of water vapor in the system. Excellent extrapolations to absolute zero can he obtained for dry air using a simple apparatus constructed from an Erlenmeyer flask, an oil manostat and a plastic syringe. Using such an apparatus, first-year chemistry students can determine absolute zero with high precision. Experlmental The apparatus (seeFig. 1) consists of a 125-mL flask fitted with a 24/40 serum stopper (Aldrich 210,145-1 or Sigma S0510) that has been drilled for a 110 OC thermometer and two short leneths of 3-mm Pvrex tubine. The manostat is a 10-cm length of 8-mm Pyrex tubing placed in a test tube or small flask of mineral oil or dibutvl~hthalate.but not water. The manostat fluid may be colo&d. The syringe is a 60-mI. (Becton-IXckinson lt5663) ~ l a s t i csvrinae with l-mL arad"ations. The syringe'kd manostat t i b e &e connected tb the flask usina flexible tubina. - A small amount of Drierite is added to the flask. In use, the entire flask and most of the syringe are lowered into a water bath starting a t 10-15 O C . After a few minutes for thermal equilibration, the syringe is set to an initial reading of about 5 mL, and, with the manostat fluid levels equal, the initial temperature of the air in the flask is read. The plunger is then retracted 2 or 3 mL, raising the inner
-
-
fluid level in the manostat, and the water bath is heated slowly. As the air temperature in the flask rises, the manostat level falls, and the air temperature is recorded a t the point when the fluid levels (pressures) are again equal. The syringe is then retracted for the next reading. The last reading should be taken a t about 65 "C. Finally, the available volume of the flask is determined after dismantling the apparatus. Results The data from a typical student experiment (student F in the table) are shown in Figure 2, in which the Celsius temperatures are plotted vs. syringe volume readings. This plot shows the thermal lag usually encountered in the first few data points. Temperature is shown on the vertical axis because i t is taken to be the dependent variable in this experiment. All of the results from one class are summarized in the table. The slopes, intercepts, and their standard deviations were obtained by linear regression using temperature as the dependent variable and total volume (syringe plus flask) as the ~redictor.The class averwe of absolute zero was -276 "C (n = 20 ' C ) .
Students should be rewired to plot their svrinae volume vs. temperature readings..~hefirs; few readings 4 1 1usually
be erratic because of thermal lag and should be neglected in Summary of Student Results number of poims slope c data ph relected' ("CImL) (slope)
Student
B C
12 19 18
2 3 2
D
22
E F G
17 16 18
-
A
"absolute zero" ('C)
(intcpt) 10 10 4 3 4 4 6
0 2 2 3
1.90 1.82 1.63 1.60 1.84 1.88 1.91
0.06 0.05 0.02 0.02 0.03 0.03 0.03
-305 -290 -246 -255 -266 -274 -294
-
-
-
-278
0
class
average
(n = 20)
"Data at me Winning of me expimem mat are more than two standard devlationr from the regression line.
104 5
J Figure 1. Charles's law apparatus.
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Journal of Chemical Education
10
15
20
25
SYRINGE VOLUME (mL) Figure 2. Charles's law data
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35
the extrapolation. The student data in Figure 2 illustrate this. The rest of the data should then be plotted as temperature on an expanded scale vs. total volume. Alternately, a computerized least-squares program will provide the intercept. Success with this apparatus requires some care in retracting the syringe plunger and reliable connections. In addition, the air in the flask and connecting tubing must be absolutely dry. This problem can be eliminated by drying the apparatus completely before assembly and also adding a small amount of Drierite to the flask. Students should be cautioned regarding the scalding hot water required to achieve a temperature of 65 'C in the flask. However, by the time the higher temperatures are reached, the students are very familiar with the apparatus and procedures.
The experiment not only illustrates Charles's law quantitatively but also emphasizes the means by which constant pressure is maintained for each of the V-T readings. Students are enthusiastic about the experiment because reliable data are easy to collect and their values for absolute zero are of high quality. Literature Cited 1. R m ,D. J. Chem. Educ. 1987,64,112. 2. Carney, G. D.; Kern, C. W .J. Chem.Educ. 1919.56.823. 3. Murdcxk, H.K.: Hawlhome, R.M., Jr. J. Chem. Edue. 1975.50.528. 4. Hall. P. K. J. Cham. Educ. 1987,64,969. 5. Chirpich. T. P. J. Chem. Edue. 1977.54,378. 6.Markow, P. 0.J. Cham. Edue. 1980.57.307. 7. tipstein, L. Loborofory Monmlfor Chemistry; Wadswiorth: Belmont, CA. 1986: p 85. 8. Mills, J. L.; Mitchell, R. ti. General Chemistry E x ~ w i m m f a Morton: : Endwood. CO,
Volume 66 Number I2 December 1989
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