Determination of the Molar Mass of Gas HKBU

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					                                      Experiment 5
 Microscale Gas Chemistry: Determination of the Molar Mass of Gas


The molar mass (MM sometimes called the gram molar mass, or the molecular weight)
of a gaseous compound can be calculated if the following are known: the mass, volume,
temperature, and pressure of the gas. In this experiment, accurate determination of the
molar mass of almost any gas can be accomplished within a few minutes using 60-mL
plastic syringes. The mass of a plastic syringe is determined with no gas present and
then again with the syringe filled with air or a sample of the pure gas. Two methods for
calculating the MM of the gas from the Ideal Gas Law can be performed.


(a) Molar Mass of Gas Device (syringe described below, the nail and a Latex syringe

1.    Pull the plunger of a new syringe outward until the volume of air in the syringe is
      at least 54 mL.
2.    Mark the location of the plunger's
      shaft that is level with the top of the
      syringe barrel as shown with the
      'hole' indicated in the figure.
3.    The mark should be near the centre
      of the plunger shaft.
4.    Remove the plunger.
5.    Hold a finishing nail with the point
      in a flame until it is hot.
6.    Use the hot nail to melt a hole
      through the centre of the plunger’s shaft near the marked location.
7.    Remove the nail and allow it to cool to room temperature.
      (The complete Molar Mass (MM) device consists of the syringe, the nail and a
       Latex syringe cap.)

                                         Expt. 5-1
(b) Determination of Molar Mass of Air

1.   Insert the plunger fully into the device.
2.   Snap the Latex syringe cap onto the
3.   Pull the plunger outward so that the nail
     can be inserted through the hole in the
4.   Two people are needed for this
     maneuver; one pulls the plunger back
     while the second inserts the nail through
     the hole.
5.   The nail should rest across the mouth of the barrel while holding the plunger in
6.   Measure the mass of the device on an analytical balance.
7.   Remove the nail and release the plunger. It should return to its former
     empty-syringe position inside the barrel.
8.   Remove the Latex cap and fill the syringe with 60 mL air.
9.   Insert the nail through the hole, discharge excess air until the nail rests across the
    mouth of the syringe and cap with the same Latex syringe cap. (Important! You
    must use the same nail and Latex syringe cap later in the experiment!)
10. Using an analytical balance, determine the mass of the MM device filled with air.

(c) Determination of Molar Mass of Carbon Dioxide

1.   Discharge the air.
2.   Transfer the gas to be studied to the MM syringe using a short length of Latex
3.   Discard the first 3–5 mL gas that is
     used to purge the air from the transfer
4.   Transfer slightly more gas than is
     needed so that the plunger hole is at
     least 1 mm beyond the top of the
5.   Insert the nail, remove the Latex tube,
     push the plunger inward until the nail

                                        Expt. 5-2
     rests across the mouth of the syringe barrel, and recap the syringe with the same
     Latex syringe cap used earlier.
6.   Determine the mass of the MM device plus the gas.
7.   Record the volume of the gas in the syringe by reading the volume from the inside
     edge of the rubber diaphragm inside the syringe barrel.
8.   Record the temperature and barometric pressure.


I. Good results require accurate values of temperature, pressure and syringe volume.
   The molar mass (MM) of the gas can be calculated from the ideal gas law:

                                     PV = nRT
                                     PV =     RT
                                     MM =
       R = 8.314472 m3 PaK-1 mol-1

II. Alternative method can be used to calculate the same experimental data by using the
    following MM ratio calculation. The equation for MM for a gas, as derived above,
    can be converted to a ratio for two gases, one of them being air. The MM of air is
    28.964 g/mol
                                              m A RT
                                     MM A =
                                     MM gas m gas
                                     MM air m air


Mass of dry syringe:                               g
Volume of gas (in dm3):                            L
Mass of syringe and carbon dioxide:                g
Temperature of room                                 C_______________K
Mass of carbon dioxide:                             g
Barometric pressure of room (in kPa)               kPa

                                       Expt. 5-3
Data Treatment

1. Determine the MM of a gas using both calculations. What are the proper units?
2. Which result most closely matches the actual MM (44.0 g/mol)?


1. Would a syringe filled with nitrogen, N2, have a greater or lesser mass than the
   syringe you filled with CO2?
2. Why does the first 3–5 mL gas from the transfer tube have to be discharged?
3. Would you get the same results for MM if you used a syringe with a nail hole in a
   different location?
4. For which calculation (I or II) is it necessary to know the mass of the empty syringe
   with no gas present?
5. Why did you measure the mass of the “empty” syringe with the plunger extended
   rather than pushed in?



                                       Expt. 5-4