Molecular Weight Determination by an Improved Temperature-Monitored Vapor-Density Method Douglas J. Grider, Joseph D. Tobiason, and Fred L. Tobiason Pacific Lutheran University, Tacoma. WA 98447
Several methods for determining the molecular weights of volatile (1-4) and nonvolatile (5, 6) liquids are found in undergraduate laboratories. Two methods employ freezingpoint depression ( I ) or surface tension (5). The others involve a manipulation of the ideal gas equation, PV = nRT = gIMRT (1) where M is the molecular weight, R the gas constant, P the barometric pressure, T the absolute temperature, V the volume of the container, and g the mass of the vapor in the container. Methods that vaporize a material to fill a known volume at a given pressure and temperature are generally referred to as vapor-density methods (2-4). The Dumas bulb method (2, 3) is used for molecular weight determinations in many freshman chemistry lahoratories. I t is less time consuming and experimentally less difficult for beginning students than other vapor-density methods such as the Victor Meyer (3, 7) and Lumsdeu ( 3 ) methods. A small amount of liquid is placed in aDumas bulb with known weight and volume, which is then submerged in a hot-water hath. When the liquid has completely evaporated, as observed by the disappearance of the Schlieren jet, the hulh is immediately sealed, cooled, and weighed. Waterbath temperature and atmospheric pressure are recorded, and the vapor mass is calculated. This method assumes that
the vapor upon sealing the hulh is a t the same temperature as the water hath, and the pressure inside the bulb is equalized to atmospheric pressure. I t alsoassumes that the Schlieren jet disappears precisely when the last drop of liquid evaporates. The mass determination experimentally limits both precision and accuracy, thus a major problem with vapor-density methods is the assumption that the mass one is measuring is indeed the mass of only the vapor. Consequently, a molecular weight determined by this method is very sensitive to time of hulh removal. Many modified forms of the Dumas hulh method have been used, some of which have employed a different technique to determine the moment of complete evaporation. Some laboratories use aluminum foil with a pinhole over an Erlenmeyer flask. At Pacific Lutheran, students use a volumetric flask and a Teflon stopper punctured with a small hole. Instead of a Schlieren jet, the student either judges complete evaporation or uses a timing factor, developed from previous runs. When evaporation was judged by eye, there was often a 10 to 25% error found in molecular weights. When the students were given a timing factor based on the characteristics of their particular liquid, the experimental values were consistent but still not satisfactory. An improvement can be made on these modifications that
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not only pinpoints the moment of rumplete evaporation hut also allows a n exact measurement of t h e vapor temperature a t the timeof removal from the water bath for weiehine. T h e u s e o f a thermocouple inside t h e flask allowsaneasy and safe wav t o monitor theexact t i m e a t which the last d r o ~ oliauid f evaporates a n d gives direct access t o t h e temperature of t h e vaDor. I n this DaDer. a n i m ~ r o v e dmolecular-weiaht-determ h a t i o n m e t h d is presented t h a t is based on a-temperature-monitored v a ~ o r - d e n s i t y(TMVD) method a n d developed b y following liquid vaporization from thermocouple response-time curves.
FLON PLUG PIN K~E-
RING STAND WITH CLAMP
Experimental Samples Alfa products reagent-grade methanol, acetone, and chloroform; Aldrich reagent-grade methylene chloride, cyclohexane, and 2-hutanane; and Aldrich spectrophotometric-gradeethyl acetate were used as samples. Equipment Figure 1 is a schematic drawing of the experimental apparatus. A in. hale for gas expansion and s larger hole were drilled in a Teflon plug.' A tight-fitting stainless steel Cole Palmer Y J I 703 thermocuuple prohe wen inserted thnrugh the larger hole and con. nected to a Cole Palmer Digitec M
