JOHN J. ALEXANDER Univwsity of Cincinnafi Cincinnati. 45221
A factory worker measured the contents of a partially filled 55-gal. organic solvent drum on a day when the temperature was 25'C and atmospheric pressure was 740 mm, and found that 25 gal. of liquid remained. After the drum was tightly resealed, a fork lift operator dented the drum reducing the interior volume to 45 gal. If the vapor pressure of the organic solvent a t 25°C is 500 mm, what is the total pressure inside the drum after being dented? Answer The pressureof the air within the drum will increase since its volume has been reduced. Temperatureis a constant. There are two gases within the drum causing the total pressure Pwithin drum = Padvent + Psir The total pressure within the drum must have been the same as atmospheric pressure at the time it was closed
+ Pair
Patmosphere = Paolmnt
140 mm
=
500 mm
+ Pair
P.j, = 240 mm Initially, the volume of the gas phase was 30 gal Vdrvm = V.,l,.t
+ Veaa ghaue
55 gal. = 25 gal.
+ V,
,h,
V,, ~ h - = 30 gal. finally, the volume of gas phase is 20 gal Vdrum = V,dvent
+ Vgas phase
45 gal. = 25 gal.
+ V,,
,h,
V, ,,haso = 20 gal. The volume of gas space above a liquid does not influence its equilibrium vapor pressure; therefore, the pressure of the solvent remains 500 mm. The oressure of the air. however. increases as a result of lower volume according to Boyle's Law Pigr,-,= 240 mm X -30 gal- 360 mm 20 gal
therefore
Pressure Calculations Charles W. Schultz Glassboro State College Glassboro, New Jersey 08028 Freshman courses almost invariably deal with the properties of gases and liquids. This question requires students t o be able to distinguish two kinds of pressure: Boyle's Law pressure of a gas (which depends on volume) from the equilibrium vapor pressure above a liquid (which does not). Since the properties of gases and liquids are often presented in separate modules, these concepts are seldom juxtaposed. This problem requires intellectual behavior a t the cognitive level of analysis.
Erratum
In the question concerning statistical tests in the October, 1911issue the followingcorrections should be made. The editor isgrateful to Mr. Walter G . Zinman of Syosset,New York, and Dr. Robert J. Merrer of Western Connecticut State College for pointing these out. 1) f a t the 95%confidence level for (nt - 1) = 3 and ( n , - 1) =4 is 9.12 (not 6.59) 2) Smmbind = l [ h - 1)S12 (n2 - 1)S22-l/(n~+ n2 - 2)11/2
+
Volume 55, Number 8, August 1978 1 515
