J. DUDLEY HERRON Pvrdue University West Lmfayene, Indiana 47907
The Variation of the Mass of a Gas with Its Pressure David E. Hopkins Gloucester High School Ottawa, Ontario, Canada KlJ7N8 In a previous note1 the construction of a weight-of-air cylinder, from an empty aerosol can, is described. If the brass core from a tire valve stem is soldered into the enlareed outlet. rather than a length of tubing, then this apparatus may hd used to demonstrate the linear dependence of the mass of a gas on its pressure and to determine atmospheric pressure and a value for the gas constant (R). The insert is unscrewed from the valve stem and a short length of flexible tubing ( n g o n * or rubber) slipped over the stem, a screw compression clip on the flexible tubing serves as an efficient tap. The apparatus is evacuated usinga rotary oil pump and itsmass determined. The insert is replaced and the can is pressurized using a foot or hand pump. The pressure of air in the can is measured using a tire pressure gauge and the mass of gas determined. A typical eraoh of mass versus oressure is shown. the different svmbols represent different runs. "
A
Since the (room) temperature is constant a least-squares straight line may he drawn through the points in the graph. The value of A can be obtained from an intercept and the slope gives the value for VMIRT. From the graph shown A = 13.2 p.s.i. (barometric value = 14.5 p.s.i.) and VMIRT = 0.0417 g p.s.i.-I with a root mean square deviation of 0.035. If values of V = 0.522 1, M = 29.0 g mole-' and T = 297°K are suhstituted into theequation for VM/RT then thisgivesR = 0.0840 l-atm OK-' mole-' (theoretical value = 0.0821 l-atm OK-' mole-'). This procedure could obviously be repeated with any compressed gas, and since only the value of M in eqn. (4) varies with the gas, the slope of the line will vary hut the intercept on the horizontal axis will he unchanged.
The Identity of Chemical Substances: A First Laboratory Experiment for Elementary Chemistry Students J a c k E. Fernandez University of South Florida, Tampa, 33620 One of the first chemical nrohlems that earlv chemists faced was establishing the identit; of a pure substance. The question m~ahrha\,e been ~ u t "How : can I determine if this vellow metal is gold?" he modern chemist still asks the every time he prepares a new compound. The experiment helow poses this question and provides an enjoyable, simple, and instructive experience through which to introduce the beginning high school or college student to chemistry during his first laboratory period.
MASS OF G A S IN CAN (PI
The Experiment
For an ideal gas PV = nRT where the symbols have their usual meaning. Since
(1)
P=G+A (2) where G = the tire gauge reading and A = atmospheric pressure and n = m/M
(3)
where m = the mass of air in the can, and M = the apparent molecular weight of air (= 29.0 for dry air), eqn. (1) hecomes VM m=-(G+A) (4) RT 'Hopkins, D. E., J. CHEM EDUC., 51,425 (1974). 718 / Journal of Chemical Education
We give each student two test tubes each containing a pure solid substance, and we ask him to determine whether the suhstances are the same or different. We give him no instructions except in the use of water, dilute acids and bases, bunsen burners, magnifying glasses, and other available rommon substances and pieces of apparatus that we wish to d a c e at his dis~osal.After a brief neriod of confusion most students reallz; that nearly any te& that shows a d~fference In behavior establ~shesthat the two substances are different. but that failure to demonstrate a difference does not provd that the substances are identical. In the ordinary 3-hr laboratory period students are able to examine several nairs of substances. We have found it best to use quite different substances in the first pair. Examples of simple first pairs are sucrose-sodium chloride. sucrose-henzoic acid, and sodium chloride-sodium carbonate.~hesecond pair might then be more similar in their behavior; examples are sodium chloride-potassium chloride, or perhaps even a pair of identical samples. Such pairs are ideal for demonstrating that two samples are identical only if all of their properties are identical including those that the student is not aware of.
