molecular weight experiment

GRAM MOLECULAR WEIGHT OF CARBON DIOXIDE

WILS BEHGSTHOM, M.S.

and

MCIRLYS HOWELLS, Ph.D.

Carbon dioxide occurs as a variable component in the atmosphere. It is formed by the decay, fermentation, and combustion of organic matter. In this experiment, carbon dioxide will be produced by reacting marble chips, predominantly CaC03, with hydrochloric acid. To obtain dry carbon dioxide, the gas is bubbled through a concentrated sulfuric acid which acts as a dessicant and collected in an Erlenmeyer flask.
The procedure involved in the determination of the molecular weight of carbon dioxide is called the gas density or vapor density method. In some texts it will be referred to as the Dumas method named for the Frenchman who is given credit for originating the method. It is based on the principle that equal volumes of gases contain the same number of molecules at the same temperature and pressure. This principle represents Avogardo’s Law which has been used to define a standard molar volume of any gas, 2 2 . 4 liters, at 1 atmosphere pressure (760 torr) and 273 K ( O oC ) . The ideal gas law, PV = nRT when solved for volume at STP defines this standard volume. If n, number of moles, in the ideal gas law is redefined as mass of gas/gram molecular weight (GMW) the formula can be rearranged to solve for the gram molecular weight by using measured values obtain in the laboratory for any PV gas sample. The variables in the formula are mass (grams), R - gas law constant (0.0821 liter-atm/mole-K), T - temperature (degrees Kelvin), P - pressure (atmospheres), and V - volume
(liters). Alternatively, the volume of gas measured at laboratory conditions could be corrected to the volume which it would occupy at STP. A simple proportion relationship would then be used to
mass of collected aaS = GMW volume occupy at STP 22.4 liters solve for the GMW.

Procedure
Take two clean, dry, marked Erlenmeyer flasks and obtain mark the bottom position of the stopper with a piece of tape. Weigh the flasks to the nearest 1 mg and record the weight. Record the laboratory temperature and pressure. Using CRC Handbookof Chemistry and Physics, record the density of air at laboratory conditions.
~ rubber stoppers that fit each tightly. Stopper the flasks and
In the fume hood, set up the apparatus as shown in the
following diagram. This will require that a number of right angle
glass bends must be produced by you. The student should read
glass cutting (830 - 8321, glass polishing (835 -836) and glass
bending (840 - 841) in the Chemical Technician's Ready Reference
Handbook, CTRRH. Remember to lubricate glass with glycerin
before inserting it into the tubing or stoppers. Place 40 g of
marble chips in a generating flask and place approximately 30 ml
of concentrated sulfuric acid, H2SO4 , in the bubbler bottle.
Be sure to check all of the rubber and glass tubing for
constrictions or blockage as the apparatus is assembled. Use a
a paper punch and 3 by 5 index card to make the paper cover for
the collection flask. Making sure that the screw clamp is closed
between the funnel and generator, add approximately 50 ml of
6M HC1 to the funnel. The liquid level in the funnel should never
exceed three-fourths of its total volume capacity nor should it be
allowed to drain completely since air would enter the generator.
Record in your laboratory notebook the handling precautions, and
the spillage and disposal procedures for all chemicals used in
the experiment from MSDS notebook.
Opening the screw clamp slightly, allow the HC1 solution to
pass from the funnel onto the marble chips. A moderate rate of gas
generation should be maintained and can be monitored by watching
the bubbler bottle. The generator should be carefully shaken
occasionally to avoid the'use of an unnecessary excess of HC1.
Permit the gas to flow for 5 minutes to ensure the displacement of
air in the apparatus. Touch the bottom of the generator flask and
record the temperature effect. Remove the paper cover and delivery
. tube from the collection flask and quickly insert a rubber stopper
to the marked position. Insert the delivery tube into a second flask
to collect a sample while weighing the first flask to the nearest
0.001 g. Alternate sample collection in this fashion until a
constant weight (50.005 g) is obtained for both flasks. When
all mass measurements have been completed, fill each of the
flasks with water to the marked position and measure the volume of
water using a graduated cylinder. To disassemble, first loosen
stopper on the generator flask to avoid siphoning concentrated
sulfuric acid into the generator. Drain the HC1 from the funnel
into a beaker and use proper disposal procedures for excess acid in
the beaker, generator and bubbler bottle. Remember that
concentrated solutions are always poured into more diluted solutions.
To determine the weight of carbon dioxide it is necessary to
calculate the weight of air in the flasks at the initial weighing.
The density of air at laboratory conditions is multiplied times
the volume of the flask to obtain mass of air. The mass of air is
then subtracted from the initial flask weight to obtain weight of
the stopper and flask. This is used to determine the mass of carbon
dioxide collected. Using one of the two methods outlined in the
introduction, calculate the gram molecular weight of carbon dioxide
for each trial and the average value. Calculate the relative error
for your results.
Exchange two clean, dry 250 ml flasks at the stockroom for
two filled with samples of unknown gases. Do not disturb the
'stopper or warm the flask unnecessarily by handling. Mark the
position of the stopper and weigh to nearest 0.001 g. In the hood
remove the stopper and displace the unknown gas with laboratory
air by use of the aspirator. Replace the stopper and weigh the
flask of air accurately. Fill the flask with water to the mark
and measure the volume using a graduated cylinder. Repeat the
calculations that were necessary to determine the gram molecular
weight of carbon dioxide.