Lab 13(1)

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Lab 13: Determination of Molar Mass Using the Dumas Method Jernellee Palacio Chemistry 101 Professor Floruti October 12, 2023
Introduction The purpose of this lab is to determine the molar mass of a volatile liquid using the Dumas method. In the Dumas method, a volatile liquid is vaporized we then gather the temperature, mass, volume, and pressure. P represents pressure of the gas, V represents the volume of the gas, n represents the number of moles of the gas, 0.08206 (L*atm/mole*K) represents the ideal gas constant, K represents temperature in kelvin. These measurements are used to determine the molecular weight of the given liquid using ideal gas law (PV = nRT). To find the molar mass in the ideal gas equation, first we isolate the n. In this experiment, n the number of moles in a sample of gas is equal to the mass of the sample (m) divided by the molar mass. N=m/M. Therefore, the new equation is PV=mRT/M. Solving for M results in M=mRT/PV. Using this equation, we will determine the molar mass of a volatile liquid. Abstract In this experiment, we measured 2-3 ml of an unknown volatile liquid into an Erlenmeyer flask. Then we cover the flask with aluminum foil, then tighten the foil with the rubber band around the neck of the flask and stick the tape to the side of the flask. We then heated the flask and all its contents until all the liquid evaporated. We then determined the mass of the vapor by subtracting the mass of the empty flask from the mass of the flask and vapor. The volume of the vapor was measured by measuring the water needed to fill the flask. The density was measured by dividing the mass by the volume of the vapor. Finally, the density, temperature, given pressure were all used to determine the molar mass of the unknown liquid. Materials
Florence or Erlenmeyer flask (125 mL or less), wire gauze square, iron ring, ring stand, beaker (300 mL or larger), bunsen burner or hot plate, clamp, aluminum foil, rubber band, Tape, Thermometer, Analytic balance. Methods Proper lab PPE -lab coat, goggles, long pants, closed toe shoes- must be always worn. Volatile organic compounds are flammable handle with care. In a small flask, take a small sample of the liquid back to your work bench. With gloved hands, cover the mouth of a dry, clean flask (125 mL or less) tightly with a piece of aluminum foil, then tighten the foil with the rubber band around the neck of the flask and stick the tape to the side of the flask. Measure the mass of the flask with all accessories in place, this will represent mf. Untie the rubber band and open the foil to add about 2 to 3 mL of the unknown liquid into the flask, then seal the flask with the aluminum foil and rubber band. Puncture a pinhole in the center of the foil, then put the flask in a water bath all the way down to its neck, make sure the flask is straightened to it boils evenly, in the water bath and clamp the flask to the ring stand. Heat the water bath slowly till water starts boiling, boil until all liquid evaporates. Sketch a picture of your set up as the water boils. Remove the flask from the water bath and put the tape on the pinhole. Wipe the outside of the flask dry and cool it to room temperature. Record the mass of the flask. Measure the mass of flask with water in it together with all accessories. Using the mass and density of water, which is 1.00 g/mL, calculate the volume of the flask. Data Collection- Dumas Method Data Table Unknown #: 44 Measurement Trial 1 Mass of flask and accessories, mf 85.6274g Mass of the flask, accessories, and condensed liquid, 86.1306g
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mc Mass of the liquid, ml=mc-mf 0.5032 Mass of the flask, accessories, and water, mw+f 233.22 Mass of water, mw=mw+f-mf 147.5926 Volume of flask 147.9365m Barometric Pressure 64.6 C Temperature of boiling water 353 K Actual Molar Mass of sample compound (get from instructor) 32.042 Calculations 86.1306g-85.6274g= 0.5032 233.22-85.6274= 147.5926 80 C+ 273= 353 K Pre-Lab Question 1. Why do you think we don't have to worry about sample evaporating from the flask during our heating process? Knowing the ideal gas law, gases expand to fill the container. Since the flask is sealed, the gas will not escape instead fill the volume of the flask. 2. How does your calculated molar mass compare to the actual molar mass? Conclusion

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