Boyle's Law Experiment R i c h a r d A. Hermens Eastern Oregon State College. La Grande, OR 97850
Experiments i n general chemistry should have certain qualifications which would allow t h e m to fall i n t o a class of "ideal" experiments. A list of these qualifications is (1) meaningful to t h e student, (2) applicable to the subject matter of t h e course. (3) safe to perform, (4) low cost, (5) simple to perform, (6)easy t o set u p a n d maintain, (7) able to obtain reproducible results, (8) capable of being performed ih a reasonable length of time. Boyle's Law experiments commonly found i n laboratory manuals require a set-up more complicated t h a n is desired a n d typically use a manometer for each laboratory station. M a nometers a r e cumbersome, rather expensive, a n d bulky to store. Apparatus The experiment described here involves a minimum of equipment and can be used for a Royle's Law experiment. The equipment required is (1) thermometer, (2) glass tube, 3 mm ad.. 30 cm in length with a slug of mercury in it, (3) 30-60-90 plastic triangle. Stockroom a&stants can prepare the gas law apparatus in advance, and with care the apparatus will last for years. The apparatus is prepared by sealing a 3-mm glass tuhe at one end. A slug of mercury is inserted in the glass tube about midway with a metlieine dropper which has been drawn to form a capillary. It must he just small enough in diameter so that it will fit in the 3-mm tube and long enough to reach midway. Theamount of mercury placed in the tube is not critical hut should be between 7and 10cm in length for goad renults. The mercury traps air between it and the sealed end of the tube and behaves as a frictionless (but not weightless) piston. The tube can be attached to a thermometer hy means of ruhher bands. These can be made by cutting washers from %-in. latex tubing. The thermometer is used to measure the leneth of traoped . . air in degrrc un~ts.SCPFigure I . Ccrr must be taken In avold jarring the tube when in a \trtical pmiticen. heraurc i t rnayrauseThe mercury 10 separate #or h ~ n m ili~placerl, e thus rummg the exprrimrnt.
Bbyle'r Law Data Rcxiprmi Length of Trapped air Tube Position
(degree marks)-' X lo2
Horizontal 0" Open-end up 90' Open-end down 90' Open-end up 60° Open-end down 60' Open-end up 30' Open-end down 30'
1.59 1.83 1.36 1.80 1.39 1.71 1.47
Ressure (calculation) Pa!" pat
+ Hgb
- Hg + (Hg sin 60') Pa, - Hg sin 60') Pa, + (Hg sin 30') Pa, - (Hg sin 30") Pat
Pa,
Pressure (trapped air1 675.7 780.0 571.4 766.0 585.4 727.8 623.6
P., - atmapheric presswe a Hg = length of mercury slug - 104.3 mm
versus IN. yields and R2 = 0.9997. Figure 2 demonstrates a plot of the data. This is comparable to student results. Conciuslon T h i s experiment fulfills all of t h e eight qualifications mentioned earlier. I t is safe to nerform. t h o u e h care m u s t he taken t o avoid mercury spillage by using a n inexpensive plastic t u b to contain t h e aonaratus durine t h e e x ~ e r i m e n tT. h e tubes can h e stored from year t o year in&lockplastic bags t o prev e n t mercury spillage a n d to eliminate mercury vapor. S t u d e n t s a r e able t o perform t h i s experiment a n d m a k e t h e i r calculations in one three-hour laboratory period. ~~
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Boyle's Law S e w n measurements arc mndr ro complete the &,sle'i Law e x . perimrv,t.'l'hest!ldent meaturv. the length uftrapped dtr { I . , in the tube while the tuhe ii horizmtal. The sttdent s h < d dlw made nware of the relationship between the length of trapped air and itsvolume. The atmosoheric oressure reading should be available for use during the rrmninder d the experimrnt. The nrxl mcacurrmrnt tube made is the ivngth of the slug penend h w n . and the length c,itrapped air is agam n w a s u r d Thr third nwnsuremcnt is madr with the tube in the vertwal pmition. open end 1111. Thp uther four mrawremmls of trapped air are made while the tuhe is olnced at :Itlo n d 60' anrlw, wtth the aid ( f a :X-tXI-Wtriangle. Each measurement is made with the open end up and open end d o h . The effective pressure of the mercury slug at these angles is given by
Effective pressure = (Hg length) sin O During all of the measurements care must be taken not to touch the glass tuhe in the vicinity of the trapped air since a change of temperature will cause serious errors. The table shows typical data that are obtained by the apparatus. The difference in the P-L product between the lowest and highest values is 1.4%. A linear regression-analysis on the seven points of P
764
Journal
of
Chemical Education
Figure 1. A hwmometer is used 10 measure the lengm of napped air in degree units.
Figure 2. Plot of pressure versus lllengm
