Learning Center
Plans & pricing Sign in
Sign Out



									                 Lab-04: DETERMINATION OF THE MOLECULAR MASS OF A VAPOR (GAS)

Introduction: In this investigation you will place a small quantity of a volatile liquid in a flask and heat it in water until
    the liquid has been vaporized completely. As the liquid vaporizes, excess vapor will be driven out of the flask.
    When the flask cools, the vapor that remains inside it will condense, and liquid will again appear inside of the flask.
    This liquid is the amount of vapor that remained inside the flask. After you have determined the mass of the
    condensed liquid, the temperature from the vapor (from the temperature of the water bath), the volume of the flask,
    and the barometric pressure (from the classroom barometer), you can combine Boyles’ and Charles’ Laws to
    calculate the volume of the gas at STP (Standard Temperature and Pressure). When combined, Boyles’ and Charles’
    Laws state:
                                                        (P1V1)/ T1 = (P2V2)/ T2
    where P = pressure, V = volume, and T = temperature. The molecular mass of any gas is 22.4 liters per mole. You
    will use this fact, the volume of the gas at STP (calculated with Boyle’s and Charles’ Laws combined), and the mass
    of the condensed vapor (weighed) to find the molecular mass of the vapor. When the amount of gas is taken into
    account, the ideal gas law formula can be used. Ideal Gas Law: PV = nRT.
Objective: To find the molecular mass of a vapor by determining the mass of a sample of vapor and
    its volume corrected to standard conditions of temperature and pressure.
Materials: Balance, 5-cm square of aluminum foil, 125-ml or 250 ml Erlenmeyer flask, pin,
   burette clamp, ring stand, ring, 400-ml or 600 ml beaker, Bunsen burner, safety glasses, unknown liquid,
   wire gauze, thermometer, 250-ml graduated cylinder, barometer.
Procedure: CAUTION: Conduct this investigation in either a fume hood or a well-ventilated room.
  1. Mass the aluminum foil and the 125-ml or 250 ml Erlenmeyer flask
  2. Add about 3 ml of the unknown to the flask, place the foil securely over the mouth of the flask
     smoothing it out very well, and make a tiny hole in the foil with a pin.
  3. Clamp the flask to the ring stand and suspend it in a 400-ml or 600 ml beaker over the burner, tilting the
     flask slightly so the liquid can be seen more easily. Pour enough tap water into the beaker so that water
     completely surrounds the flask, but does not touch the foil. Check your setup with your instructor before
  4. With your instructor’s approval, heat the liquid until the last trace is gone from inside the flask.
  5. When all of the liquid has evaporated, including any which may have condensed inside the neck of the
     flask, use the clamp to remove the flask from the water bath. Place the flask on the counter and allow it to
     cool. Be sure to remove the clamp before letting go of the flask or it will topple over. Wait until the flask
     has completely cooled before proceeding any further.
  6. After the flask has cooled, examine the cap to make certain there are no water droplets on the outside
     surface. Wipe it and the rest of the flask dry if necessary.
  7. Mass the flask, the foil cap, and the condensed liquid.
  8. Repeat steps 2 through 7 several times if time permits
  9. Next, find the volume of the flask by removing the foil cap from the flask and filling the flask to the brim
     with tap water. Measure this volume by pouring the water into the graduated cylinder.
Results: Make a data table to record all observations and measurements
                                                                                            P = pressure
Calculations:                                                                               V = volume
 1. Determine the volume of the flask.                                                      n = # of moles
 2. Determine the mass of the unknown remaining in the flask.                               m = mass
 3. Calculate the molecular mass of the unknown.    (PV = nRT & M = m/n)                    M = molecular mass
 4. Obtain the molecular mass from each group & determine average & std dev.
Questions: Explain how each of the following would affect the experimental molecular mass:
 1. The top portion of the flask in not at the BP.
 2. Not all the liquid evaporated.
 3. Moisture is caught in the aluminum foil.
Lab: MW of Gas                       Name _______________________________________
Lab Data:                                                         Calculated Data:

1. Mass of dry flask plus foil: ____________           Mass of gas in flask ______________

2. Mass of flask after heating: ____________           Moles of gas in flask _____________

3. Volume of the flask: _________________

4. Temp. of the gas in the flask: __________           Molecular Mass of Gas ___________

5. Pressure of the gas in the flask: ___________________


    Group #        Molecular Mass (g/mole)
Avg Molecular
Mass (g/mole)
Std. Dev.

To top