Microscale Gas Chemistry: Molar Mass of Gas Determination.
A chemistry laboratory 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 (a) air and (b) a sample of the
pure gas.
Two methods for calculating the MM of the gas from the Ideal Gas Law can be
performed. This laboratory activity is suited for high school and university-level chemistry
students.
Materials needed for construction:
• 60-mL plastic syringe • Latex syringe cap • finishing nail, 5-cm (2 inch)
Instructions: Pull the plunger of a new syringe outward until
the volume of air in the syringe is at least 55 mL. 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
left figure.
The mark should be near the center of the
plunger shaft.
Remove the plunger.
Hold a finishing nail
with the point in a flame until it is hot. Use the hot nail
to melt a hole through the center of the plunger’s shaft near
the marked location. 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.
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
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Figure 1.
In a separate syringe or syringes, generate a gas for study. Start with carbon
dioxide; it works well. (Use 0.25 g of sodium bicarbonate and 5 ml of vinegar.)
Lubricate the rubber diaphragm of the plunger of the MM syringe with silicone oil or
spray or equivalent. 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 syringesyringe transfer using a short length of Latex tubing. Discard the first 3 – 5 mL gas that
is used to purge the air from the transfer 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. Good results require accurate values of temperature, pressure and syringe
volume. The molar mass of the gas can be calculated from the ideal gas law:
PV  nRT
m
n
MM
m
PV 
RT
MM
mRT
MM 
PV
Calculation II.
Better results are obtained for 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
MMA 
mA RT
PV
MM gas m gas

MM air m air
Experimental Results.
We have studied eleven gases and have reported the results at our
website. Excellent accuracy and precision are obtained! For carbon dioxide, Calculation #1
we reported MM = 42.5 +/- 1.4 with five measurements. This differs from the actual MM = 44
g/mol by 3.4%. With Calculation II and the same five experimental results, we obtain MM =
43.3 +/- 1.2 which is within 1.5% of the actual value!
Laboratory
Mass of
Mass of
Mass of
kPa):
data sheet:
dry syringe:
syringe and carbon dioxide:
carbon dioxide:
Volume of gas (in dm3):
Temperature of room (in K)
Barometric pressure of room (in
Laboratory Results:
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?
Notes:
1. This article appeared in Chem13 News in September, 2001.
2. This equipment and our two books 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
3 Website: http://mattson.creighton.edu/Microscale_Gas_Chemistry.html