GEORGE L. GILBERT
Uenlson Unwerslty Granwlle. Ohlo 43023
An Exercise with Boyle's Law Submitted by: Michael B. Moeller University of North Alabama Florence, 35630 Checked by: Ronald E. DiStefano Northampton County Area Community College Bethlehem, Pennsyluania 18017 Apparatus
One 1000-ml Erlenmeyer filtering flask. A piece of ruhher tubing for the side arm of the flask, with either a stopcock or a pinch clamp. One 50-1111 buret with its pointed tip hroken off. A one-hole ruhher stopper. Procedure
The huret is filled with water and mounted on the filtering flask using the rubber stopper. With the stopcock on the side arm open, the water level in the buret is adjusted to the zero position. This accomplished, the side arm stopcock is then closed, sealing the atmosphere inside. The object is to calculate beforehand the volume of water which will enter the flask when the huret stopcock is opened. This calculation can be easily set up having all pressures in units of cm H 2 0 , l atm = 1033 cm H20. The data required for the calculation are the height in centimeters of the zero graduation above the tip of the buret, ho; the distance between the milliliter graduations, c; the volume of the filtering flask, V'; and the atmospheric pressure, P'. Water will stop flowing when the pressures on both sides of the water surface a t the bottom of the huret areequal. The pressure downward is given by (1) Pi=P'+ho-c.x where x is the number of milliliters of water which have drained from the huret. The pressure exerted upward by the gas in the flask, P", can he calculated using Boyle's law. P'V' = P"V"
The new volume occupied by the gas, V", is equal to V' - x . Eauatine PI and P? and a little aleehra eives a quadratic e~uatioifor~x
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cx2-(V'e+P'+h")x+hoV'=O (3) Substituting the known values into eqn. (3) and taking the negative root gives the equilibrium huret reading after the stopcock is opened. CautionslComments
This demonstration not only illustrates a fundamental gas law hut also can he a challeneine for eeneral chem- -problem . istry students. The set-up can he shown to th;class without 584 / Journal of Chemical Education
opening the buret stopcock and then allowing a day or two for the students to come up with the answer. The equipment is simple and low-cost and the exercise could he utilized as a lahoratorv experiment a t the hieh school level. esneciallv in schools whichdo not have elaborate gas-law eduipment." For best results the water should be a t the room temperature and the atmosphere in the flask initially humidified with a few drops of water. This can be done while adjusting the water level in the huret to the zero mark. That the water vapor does not follow Boyle's law is insignificant because of its relatively small partial pressure. Students performing the experiment should he cautioned not to hold the flask in their hands during a run as this would warm the air in the flask. The actual volume of the filtration flask should he determined as i t most probably is significantly greater than 1000 ml. An actual barometric pressure reading.is not required for satisfactory results. An estimation of the expected agreement between the calculated and actual results can provide a challenging problem to students in physical chemistry. Along with the propagation of random errors, there is a systematic error due to the water's surface tension which provides an additional upward force. It is for this reason that a huret with the tip hroken off was specified in the apparatus. Removing the narrow tip t o give the buret end a larger diameter reduces the effect of surface tension. If a whole buret is used, the additional height of water suuoorted bv the surface tension can he estimated from the fo&ula for capillary rise. Alternatively, a surfactant may he added to diminish the surface tension effect.
The Color Blind Traffic Light Mathilda M. Boulanger Uniuersity of Massachusetts Amherst, 01003 "The Color Blind Traffic Light" previously reported by John F. Leflhocz' with an induction time of approximately 25-40 min frequently exceeded the alloted lecture period before the oscillatine reactions could he demonstrated. Successful modificationuof this exercise provides a fascinating lecture demonstration that stimulates student interest. and peneratrs curiosity about the study of chemical kinetics. Revetitivr color chanres induced t ~ v1.10-ohenanthrnlt:in ft.rrous sulfate complex (ferroin) from-videito blue motivates the student to consider the theow of the kinetics involved. The concentrations of reactants necessary to obtain oscillations is NOT criticaL2 Stock concentrations utilized for demonstrations range as follows: ceric ammonium sulfate, Tested Demonstrations is a monthly feature designed to present lecture demonstrations and experiments in a format convenient for classroom use. Readers interested in either submitting or checking demonstrationsshould contact the column editor. An outline of format requirements was given on page 166 of the March 1976 issue of This Journal. ~
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