K. N. Carter
A Simple Gas Law Apparatus
Presbyterian College Clinton, South Carolina
The sinlnle eas law annaratus described here can he easily used to ( a ) verify the ideal gas law, ( b ) determine t,he vapor pressure of water a t an elevated temperature, or (c) determine the number of moles of dry air under two different sets of conditions for comparison. The apparatus is shorn in the diagram. The only part not already available is easily made by closing a 10-mi length of 10-mm tubing and sealing it to a Thermometer 5-cm length of ghss rod. A thin rubher hand is Stopper placed on the rod to mark the water level in the beaker, and another is /I I1 11 placed on the tube to mark the volume at the end of the experiment. The apparat.us is arranged as shown in the diagram, using a 1000-ml beaker n~ormtedso that it can be heated with a burner. The water in the bcaker is heated slowly, with st,irring, to about 80°C and kept a t that temperature for one or two minutes. .knv air bubble ~ r o t r u d i n efrom the tube is raked off 1;-ith the bent stirring rod: The temperature is recorded, and the water is cooled to about 5OC with ice. The low temperature is recorded A.
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302 / Journal of Chemical Education
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and the band on the tube moved (making sure that the beaker water level coincides vith the band on the rod) to mark the level in the tube. The tube is removed and the volumes carefully determined. This can be done by standing the tuhe upside down on the stopper as a base on a triple beam balance. The weight of water added up to the mark can be used to measure the volume. Addition of a measured volume of water from a buret could he used instead. The effect of hydrostatic pressnre on the two volumes is determined from the distanres from the hand on the rod to each of the levels in the tuhe. Typical data:
PI = Psar - PHS VI
=
TI = P2 =
V2 =
T.
=
+ d,(converted) +
=
744 - 341 8 = 411 mm H g 5.00 mi high ttemperaturc = 332'Ii Psar - P H ~ O &(converted) = 744 - 7 4 = i 4 1 mm Hg 2.30 ml low temperature = 2iS"Ii
+
+
Using the data to verify the gas lam: V 2 (calculated) = 2.19 ml (observed = 2.30 ml). Using the experiment to calrulate the vapor pressure of water a t the higher temperature, PI is calculated and the vapor pressure determined. From the data given, the vapor pressure calculated for 79-C is 320 mm (literature value = 341 mm). Using the experiment to determine the number of moles of dry air under each set of conditions gives values of 9.36 X and 9.82 X 10-5.
