Physical Chemistry Laboratory Molecular Weight of a Vapor

The molecular weight of ethyl acetate was determined in this experiment using a formula derived from the ideal gas law and experimental measurements of mass, volume, temperature and pressure of the sample. The molecular weight was determined to be 50 g/mol +/10% (RSD). This value compares unfavorably to the literature value of 88 g/mol [1]. RESULTS AND DISCUSSION The experiment was performed using a Corning hotplate, a Mettler analytical balance, mercuryfilled barometer and thermometer, a 100mL round-bottom flask (rbf), and miscellaneous pieces of common lab equipment, supplies, and glassware. The volume of the rbf was determined by first obtaining the mass of the empty flask sitting on a cork ring (for stability) using an analytical balance. Fingerprint masses were avoided by using gloves to handle the rbf. After recording the empty rbf mass (and cork ring), the flask was then carefully filled with distilled water until the top of the water meniscus was just above the top of the rbf. This level was decided to most accurately represent the volume of the flask. Finally, the empty mass was subtracted from the filled mass to obtain the mass of the water it took to fill the flask (Equation 1). mH2O = mf mi (1) = 155.4046g – 54.9135g [4] = 100.4911g Where mH2O is the mass of distilled water to completely fill the rbf mf is the mass of the distilled water, the rbf, and the cork ring mi is the mass of the rbf and cork ring The temperature of the water was found using a mercury thermometer. The volume of the rbf was calculated using the temperature and the density of water (Equation 2). Vrbf = VH2O = mH2O/ρH2O (2) = 100.4911g/0.99701g/mL = 100.79mL Where Vrbf is the volume of the rbf (equiv. to the volume of the water contained by the rbf) ρf2O is the density of pure water at 21oC, corrected for buoyancy (0.99701g/mL) [1] mH2O is the mass of the water A 400mL beaker of water was heated to boiling. The temperature of the water bath was measured with a mercury thermometer to +0.1oC. The temperature was carefully obtained from a mid-point in the boiling water bath rather than close to the sides or bottom of the beaker. Also, the barometric pressure was obtained to +0.01cm Hg. Unit conversions are shown in Equations 3 and 4. Tvapor = TH2O Bath = 99.7oC = (99.7 + 273.15)K = 372.85K [4] (3) Pvapor = Patm = 74.19cm Hg = (1atm/76.00 cm Hg) = 0.9762atm [4] (4) The rbf was then emptied and dried using a small amount of acetone. It was shaken several times to hasten the drying. Next, a circle was drawn on the bottom of the rbf that was approximately the diameter of 1⁄2 of a dime. A quarter-size (~10 drops using a Pasteur Pipet) amount of ethyl acetate was added to the flask, then it was covered with a small (~1.25in2) piece of aluminum foil. To insure that the vapor was at atmospheric pressure and to allow the excess volume of vapor to escape, a small hole was pushed into the foil with a pin. Finally, the rbf was attached to a ring stand via a round-jaw clamp and lowered into the boiling water bath. The flask was submerged as deeply as possible in the 100mL flask to ensure uniform temperature of the vapor. After a few minutes, the solvent started to evaporate. When the solvent’s surface area reached the size of the circle drawn on the bottom of the flask, it was removed from the hot water. This small amount of residual liquid appeared to vaporize during the process of removing it from the water bath. Once the flask cooled, the aluminum foil cap was removed and the outside of the flask was quickly dried. Using the same balance and cork ring, the mass of the flask was soon determined. Again, fingerprint masses were avoided by wearing gloves. The mass was found by difference (Equation 5): mvapor = mf‘ mf (5) = 100.6580g 100.4911g = 0.1669g Where mvapor is the mass of ethyl acetate vapor mf’ is the mass of the rbf, cork ring, and vapor mf is the mass of the rbf and cork ring Finally, the vapor masses were converted to MW (Equation 6): ( ) ( ) ( )( ) ) 6 ( 79 . 100 9762 . 0 85 . 372 057 . 82 1669 . 0