GEORGE L. GILBERT Denison University Granville. Ohio 43023
Boyle's Law Derek A. Davenport P u r d u e University West Lafayette, Indiana 47907 George F. Wollaston Clarion State College Clarion, Pennsyluania 16214
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Preparation 1) From 1 atm to 3 atm. Provide a 30-ml hypodermic syringe, a 5 in. X 5 in. piece of wood or Masonite drilled so as to slip over the barrel of the syringe and press against the flange, a ring-stand with ring, and a spring platform-balance reading to ahout 30 lh. Baby scales, which are often given away as acts of propitiation, serve admirably. 2) From about 0.25 atm to 1 atrn. As in (1) except that a spring scale with a large circular dial (reading to about 10 lh) replaces the spring platform-balance. An alternative method uses the spring-platform balance plus a standard building hrick weighing 8-10 lh. Demonslratlon 1) A 30-ml sample of air is introduced into the syringe lubricated with glycerol which is then inverted so that the plunger rests on the balance plate and the woodensquare just touches the underside of the ring. A series of simultaneous volume and weight readings is taken as the syringe is intermittently forced down against the balance platform. Alternatively, the syringe may he pressed down and read by one person while a second takes the scale readings. The process is r e ~ e a t e dwith different initial volumes andlor different gases. Typical data (all obtained in five minutes) are given below: Volume
@ma)
Scale Reading (lb) HI
Air
He
dwh /.
that found in (1).Alternatively, the hrick is placed on the platform-halance and the change in its apparent weight monitored as it is raised by means of the string or wire loop attached to the piston of the syringe. By use of suitable scaling factors 1/V may he plotted against "P" over the entire twelve-fold pressure range. Remarks A syringe is designed to deliver rather than to contain a calibrated amount and there is a very small volume of unmeasured gas in the tip of the syringe and the needle. More in the interests of safety than of accuracy, a sliver of pressure tubing may be placed inside the syringe to compensate for this. With a carefully lubricated syringe, we have had no leaksge or breakage problems even a t 4 atm, hut safety glasses should b e worn, of course.
' Davenport,Derek A., J. CHEM. EDUC., 39,252 (1962). Ga(U)Ge Pressure and Absolute Pressure D e r e k A. Davenport Purdue University West Lafayette, Indiana 47907 A standard hicycle or automobile tire-gauge is inserted through a rubber stopper in such a manner that the piston of the gauge may operate inside a large test tuhe carrying a side-arm connected to a manometer and a vacuum pump. The pressure of a tire or of a refill can of "hutane"lighter fluid1 is then taken in the usual manner (practice the knack ahead of time) as the air is progressively removed from the test tuhe. The sum of the gauge pressure and the residual pressure (in the same units) is sensibly constant. When the test tuhe has been completely evacuated the gauge reading will he found to have increased by an amount equal to the atmospheric pressure. a ~ n a r a t unroduces s an admirable s h a d o w m a ~ when h The - --. placed on an ove;head projector. Readings m a q b i m a r k e d with rubber hands or else read from a superimposed projected grid.
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When the reciprocal of the volume is plotted against the scale reading straight lines result. Providing the same syringe is used, the three lines converge a t -9.0 lh. The thoughtful student should be asked to calculate the distance between the 0 and 30 ml marks on the syringe. The confusion of units adds interest. Atmospheric pressure must he supplied.' 2) A string or wire loop is attached to the plunger of the syringe. A small (5-10 ml) sample of gas is sealed in the syringe and the hook of the spring scale slipped through the loop. The two are then eraduallv. se~arated. the ring and Masonite (this . time slipped uver the pl~myer)again restraining the tlangr if the svrinrt:, nntl rendinps are taktw u s beiore. A p h t of Ill' versis scale reading is again linear hut of a slope opposite to
Davenport, Derek A,, J. CHEM. EDUC., 39,252 (1962).
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322 1 Journal of Chemical Education
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 demonstra~ionsshould contact the column editor. An outline of format requirements was given on page 166 of the March 1976 issue of this Journal.
