General Safety Precautions.
    Always wear safety glasses.  Gases in syringes may be under pressure and could spray liquid chemicals.  Follow the instructions and only use the quantities suggested.

Toxicity.
    Refer to the appropriate chapter to learn more about the toxicity of each gas.

Suitability.
    This laboratory activity is suited for high school and university-level chemistry students.
 

Construction of the Molar Mass of Gas device:

Materials needed for construction:²
• 60-mL plastic syringe
• Latex syringe cap
• pliers
• burner
• finishing nail, 5-cm (2 inch)

Using the Molar Mass of Gas device.

Materials needed for the experiment:

• Molar Mass of Gas device (syringe described above, the nail and a Latex syringe cap)
• analytical balance
• additional 60-mL plastic syringes for generating gases
• additional Latex syringe caps
• 2-cm lengths of Latex tubing, 1/8-inch (3.175 mm) ID
• other materials required for generation of each gas
• cotton balls (or ‘puffs’)
• calcium chloride (anhydrous)

     Gases prepared by our methods must be dried before use.  This is accomplished by passing the gas sample through a 3-cm length of Latex tubing that has been packed with pulverized anhydrous CaCl2 and held in place by plugs of cotton as shown in Figure 2.  The drying tube is used to connect the reactant syringe to the MM syringe, described below.

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

     Discharge the air. Transfer the gas to be studied to the MM syringe via syringe-syringe transfer using a short length of Latex tubing or drying tube as shown in Figure 3. Discard the first 3 ? 5 mL gas that is used to purge the air from the drying tube.  Transfer slightly more gas than is needed so that the plunger hole is at least 1 mm beyond the top of the syringe.  Insert the nail, remove the Latex tube and gas generation syringe, push the plunger inward until the nail rests across the mouth of the syringe barrel, and recap the syringe with the same Latex syringe cap used earlier.  Determine the

mass of the MM device plus gas contents.  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.  Record the temperature and barometric pressure.

Calculation I.  The molar mass of the gas can be calculated from the ideal gas law:

Good results require accurate values of temperature, pressure and syringe volume.  A more accurate volume determination can be obtained by filling the MM syringe with water instead of gas and using the mass of water and its density to calculate the volume of the MM syringe.  Note: The volume of the filled MM device should not change, so this measurement needs to be done only once.

Calculation II.  Better results are obtained for the same experimental data by using the following MM ratio calculation.  The equation for MM for Gas A, as derived above, can be converted to a ratio for two gases, A and B:

The advantage of this ratio method is that accurate values for temperature, pressure and syringe volume are not needed because they cancel at constant T, P and V.  In our experiments, Gas A is the gas being studied and Gas B is air.

Using Dalton's law of partial pressures and the Ideal Gas law, the MMair can be estimated from the mole fraction, X, of the constituent gases N2, O2, Ar, and CO2, which collectively account for over 99.99% of air.

Using accepted values for X,⁷ we can estimate the 'molar mass' of air:

Experimental Results.  We have studied ___ gases using this method and the results are collected in the Table.
 

Table.  Summary of Molar Mass Experiments using the MM Device.

Gas	Actual MM (g/mol)	Commercial  gas samples MM* (Calc. I)	Commercial  gas samples MM* (Calc. II)	Prepared  gas samples MM* (Calc. I)	Prepared  gas samples MM* (Calc. II)
CH4	16.0429			-	-
NH3	17.0306	17.5 +/- 0.3 (2.9%)	17.4 +/- 0.2 (2.4%)
C2H2	26.0379			25.8 +/- 1.3 (1.1%)	26.9 +/- 0.1 (3.2%)
N2	28.014	27.5 +/- 1.1 (1.8%)	28.5 +/- 0.5 (1.5%)	29.1 +/- 0.7 (3.8%)	28.6 +/- 0.4 (1.9%)
NO	30.0061	31.0 +/- 0.4 (3.3%)	31.1 +/- 0.5 (3.5%)
O2	31.999	31.7 +/- 0.6 (1.1%)	32.2 +/- 0.1 (< 1%)	30.2 +/- 1.5 (5.6%)	31.5 +/- 0.1 (1.5%)
Ar	39.948	39.9 +/- 0.2 (< 1%)	39.9 +/- 0.4 (< 1%)	-	-
CO2	44.010	43.0 +/- 2.2 (2.2%)	45.0 +/- 0.2 (2.2%)	42.5 +/- 1.4 (3.4%)	43.3 +/- 1.2 (1.5%)
N2O	44.013	43.0 +/- 1.5 (2.2%)	44.4 +/- 0.2 (<1%)
C3H8	44.0968	43.2 +/- 0.1 (2.1%)	43.3 +/- 0.5 (1.8%)	-	-
C4H10	58.1237	56.9 +/- 3.0 (2.1%)	58.3 +/- 1.0 (0.3%)	-	-
SO2	64.0628	-	-
Cl2	70.906	70.2 +/- 2.6 (1.0%)	71.8 +/- 1.6 (1.3%)

* average of five trials +/- standard deviation⁸ (% error)
 

Clean-up and Storage.
    At the end of the experiments, clean all syringe parts (including the diaphragm), caps and tubing with soap and water.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.
 

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

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

Laboratory Report Questions:

1. Would a syringe filled with nitrogen, N2, have a greater or lesser mass than the syringe you filled with CO2?
2. Why did the instructions say to discharge the first 3 ­ 5 mL gas that is from the transfer tube?
3. Would you get the same results for MM if you used a syringe with a nail hole in a different location?  Is MM an intensive or extensive property?
4. For which calculation (I or II) was 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?

Endnotes:

1 Author to whom correspondence should be addressed.  E-mail: xenon@creighton.edu

2 Website:  http://mattson.creighton.edu/Microscale_Gas_Chemistry.html

3 This equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), and Fisher Scientific.  Part numbers and links to their websites are provided at our microscale gas website (Endnote 3)

4 The Chemistry of Gases, A Microscale Approach, Mattson, B. M., Anderson, M. P., Schwennsen, Cece, Flinn Scientific, 1999, ISBN #1-877991-54-6.

5 Microscale Gas Chemistry, Educational Innovations, 2000, ISBN #0-9701077-0-6.

6 Pulverized blue-colored Drierite⁶ (anhydrous CaCl2) granules works especially well because the blue indicator turns pink in the presence of water.  Available from Fisher Scientific  07-578-3A.

7 Chemistry, 3rd Edition, McMurry, J., and Fay, R., Prentice Hall, 2000.

8 The formula for the standard deviation, s, of n values is:

  where x represents the individual MM values,  represents the mean MM and nis the number of measurements.