Determining Molar Mass by Ideal Gas Law

I. Abstract: By manipulating the ideal gas law (PV=nRT), we will be determining the molar mass of an unknown volatile liquid. Heating a flask filled with an unknown, easily evaporated liquid will allow for measurements that can be taken to work out the ideal gas equation. This lab will require knowledge of basic equations used in chemistry. Using these equations, such as density and number of moles(n), we can substitute different values into the ideal gas law to manipulate it. II. Materials: 250-mL Erlenmeyer flask Needle or pin Unknown liquid sample Barometer 1000-mL beaker Hot plate Utility clamp Aluminum freezer foil Hot mitt Wire gauze with ceramic center 1000-mL graduated cylinder III. Procedure: 1. Aprons and goggles on. Clean a 1000-mL beaker for use as a heating bath. Set the beaker on a hot plate and begin heating. 2. Clean and completely dry the Erlenmeyer flask. 3. Cut a square of aluminum foil to serve as the cover of the flask. Trim the edges so that it neatly covers the edge of the flask. 4. Weigh the empty flask with cover and record to as many sigfigs as possible. 5. Obtain your unknown liquid and record the ID #. Add 3-4 mL of the liquid to the flask. Re-cover the flask, making sure the edges are tightly crimped. 6. Punch a tiny hole in the foil cover with a needle or pin. 7. Heat the water in the beaker to boiling. Adjust the heat so that the water will remain boiling but will not splash. 8. Immerse the flask containing the unknown liquid in the boiling water so that most of the flask is covered. Clamp the neck of the flask. 9. Watch the liquid carefully. The liquid will begin to evaporate, and its volume will decrease. 10. When it appears that the flask is completely filled with vapor, continue heating for 1-2 minutes. Remove the flask from the bath using the clamp. 11. Set the flask on the wire gauze on the lab table, remove the clamp, and allow the flask to cool to room temperature. Measure and record the exact temperature of the boiling water in the beaker, as well as the barometric pressure in the lab. 12. When the flask has cooled completely, carefully dry the outside of the flask to remove any water. Weigh the flask, foil cover, and vapor with as many sigfigs as possible. 3. Perform a second trial, if the final mass of the flask and vapor is not within 0. 05g of the first trial, perform a 3rd. 14. When two acceptable trials have been performed, remove the foil cover and clean the flask. 15. Fill the flask to the very rim with water and pour it into a 1000-mL graduated cylinder to determine the exact volume of the flask. Record. IV. Data and Observations | |TRIAL 1 |TRIAL 2 | |BAROMETRIC PRESSURE (mm Hg) |763. mm Hg |763. 5 mm Hg | |MASS OF FLASK AND FOIL COVER (g) |106. 095g |106. 095g | |TEMPERATURE OF WATER IN BEAKER ( °C) and |99. 1 °C / 372. 1 K |97. 9 °C / 370. 9 K | |(K) | | | |MASS OF FLASK/COVER AND CONDENSED LIQUID |106. 406g |106. 87g | |(g) | | | | | | | †¢ Volume of Erlenmeyer Flask (determined by filling completely with water): 289mL †¢ Unknown liquid ID #: B – (ethyl alcohol) V. Analysis of Data PV = nRT Trial 1) 763. 5 (. 289 L) = n (62. 4) (372. 1) 220. 6515 = 23219. 04 n 220. 6515/23219. 04 = n 0. 0095 mol = n Mass of vapor in the flask = (mass of flask/cover and condensed liquid) – (mass of empty flask/cover) Mass of vapor in the flask = 106. 406g – 106. 095g = 0. 311g 0. 311g/0. 0095 mol = 32. 73 g/mol = molar mass Trial 2) 763. 5 (. 289 L) = n (62. 4) (370. 1) 220. 6515 = 23094. 24 n 220. 6515/23094. 24 = n 0. 0096 mol = n Mass of vapor in the flask = (mass of flask/cover and condensed liquid) – (mass of empty flask/cover) Mass of vapor in the flask = 106. 487g – 106. 095g = 0. 392g 0. 392g/0. 0096 mol = 41. 02 g/mol = molar mass AVERAGE MOLAR MASS: (32. 3 + 41. 02) / 2 = 36. 875 g/mol Theoretical molar mass of ethyl alcohol: 46. 07 g/mol VI. Conclusion (36. 875 – 46. 07) / 46. 07 (x 100) = 19. 9% = percent error The first trial was very unsuccessful while the second trial was much closer to the theoretical value. The possibilities of error within the first trial include not letting the flask cool completely to room temperature before weighing and also forgetting to utilize acetone to dry the flask before the addition of the liquid. The latter was the major difference in performance of the two trials. This experiment showed that the interference of water vapor can completely wreck the chance for an accurate measurement of volume of a different gas. It is imperative to make sure that the flask is as dry as possible or the results will show a very wrong answer. It is hard to realize the prominence of water vapor in the air until an experiment like this one shows that it takes up a relatively huge space in a given volume of air. A suggestion to improve this lab could be to place more emphasis on drying the flask with a paper towel and acetone before use.