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An organic chemist named John Dumas (1800-1884), devised a simple method for determining the molecular weights of volatile liquids. Volatile liquids are liquids which evaporate (vaporize) readily at normal pressures and temperatures. In the Dumas method (see example problem) a volatile liquid is vaporized and measurements of the vapor’s pressure, volume, mass, and temperature are used to calculate the molecular weight of the liquid based upon the ideal gas law (PV = nRT). In this experiment the Dumas method will be utilized to determine the value of the gas constant, R, from measurements made on the vapor of the volatile liquid hexane, C₆H₁₄.
 

Example When a 0.1332 g sample of an organic liquid was vaporized, the volume of the vapor was 70.21mL at 100.22 oC and 761.23 mmHg. What is the molecular weight of the substance? PV = nRT (761.23 mmHg)(0.07021 L) = (0.1332g / MW)(62.4 L×mmHg/mol×K)(373.37 K) MW = 58.06 g/mol

Procedure
  1.     Set up a hot water bath using an 800 mL beaker containing about 600 mL of water and bring the water to a SLOW boil.
         After the water is brought to boiling, the flame of the bunsen burner can be reduced.

  2.    Using a push pin, place a small hole in the center of a 2 inch square of aluminum foil.

  3.    Weigh a dry, 100 mL volumetric flask and the square of aluminum foil and record the mass.

  4.    Using a graduated cylinder, measure out about 5 mL of hexane, C₆H₁₄, and transfer the hexane to the volumetric flask.

  5.    Place the aluminum square over the center of the mouth of the flask.

  6.    Clamp the flask into the hot water bath. Caution should be used due to the fact that the hexane is flammable.
         A small flame will be sufficient to keep the water boiling.

  7.    Heat the flask with a slow boil until all the hexane has been evaporated and continue heating for an
         additional 3 minute period.

  8.    After the heating period, turn off the bunsen burner and CAREFULLY remove the flask and place it in cold
         water and allow the hexane vapors to condense.

  9.    Dry the flask and weigh it, the foil, and the condensed liquid. Record the mass.

10.    Dispose of the hexane by pouring it into the labeled waste bottle under the hood.

11.    Perform a second trial by repeating steps 3-10. If needed use another foil square.

12.   After performing two trials, accurately measure the volume of the Erlenmeyer flask by filling the flask with water
        to the mark and using a buret, fill the flask to the mouth with water. Read and record the initial and final buret
        readings to determine the  total volume of the flask.

Name_________________________________________ Hood No.___________ Date___________

(Calculate percent error based upon a value of R = 62.364 L×mmHg/mol×K.)

Calculations