EXPT 2. Molecular Weight of
Carbon Dioxide
[Key Contents]
molecule, molecular weight, phase change, sublimation, gas density,
Avogadro's principle, ideal gas law
[References]
Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.)
Ch 1. The Atom in Modern Chemistry
Ch 2. Chemical Formulas, Chemical Equations, and Reaction
Yields
Ch. 9. The Gaseous State
Chemistry for Life, Chemistry for Better Life (Kim et al.)
Ch. 3. Atoms and Molecules
Ch 7. States and Properties of Matter
[Goal]
- to learn to determine molecular weight of a gaseous molecule
from the gas density as well as using the ideal gas law. Dry ice will
be used to create CO2 gas at room temperature and 1 atm.
- to observe liquid carbon dioxide under high pressure at the
triple point
- to observe the rate of diffusion
[Background]
Atomic hypothesis and atomic weight were key ideas in Expt 1. In
Expt 2, we will focus on molecules. Roald Hoffmann, winner of the
1981 Nobel chemistry prize, said : Atoms are nice, atoms are
fundamental,
but
they
are
not
chemistry.
Chemistry
is
about
molecules, the fixed but transformable way in which atoms get
together for a while. Thus, understanding how chemical bonding of
atoms leads to molecules and how molecules behave constitute
central themes of chemistry.
To be precise, the world around us is the world of molecules
instead of atoms. Except for noble gases, such as argon and helium,
and a few elements like gold and sulfur which exist as pure
substances, atoms are found in combination with other elements like
oxygen. Therefore, determiantion of molecular weight is as important
as determiantion of atomic weight. Knowing molecular weight, one
can determine how many atoms of which elements are within that
molecule.
How can one determine molecular weight? In this experiment, we
will focus on gas molecules. According to Avogadro's principle which
played a key role in the molecular hypothesis, the same volume of
gases at the same temperature and pressure contains the same
number
of
molecules
not
atoms.
Therefore,
one
can
determine
molecular weight from gas density measurements.
Amadeo Avogadro, 1776-1856
Avogadro's principle
One can easily prepare quite pure carbon dioxide gas at 1 atm
and room temperature using dry ice, which is solid carbon dioxide.
Upon sublimation, dry ice turns into gaseous CO2 and fills a conatiner
because it is heavier than air. Once you determine the weight of a
known volume of CO2 at 1 atm and room temperature, you can
calculate molecular weight either by comparing the weight of CO2
with the weight of air in the same volume or by using the ideal gas
law.
Under high pressure, one can easily observe liquid CO2 which can
not be observed at 1 atm because dry ice turns directly to gas.
Observing liquid CO2, one can get a real sense of the phase change
through the triple point.
[Apparatus and Chemiclas]
electronic balance, tygon tubing, 250 ml flask, aluminum foil, spatula,
cylinder, thermometer, stand, clamp, nipple, hammer, screw driver,
funnel, teflon tape, cotton glove
dry ice
[Procedure]
Expt 1. Molecular Weight of Carbon Dioxide
1) Weight an empty flask covered with a glass plate.
2) Transfer 1 spoonful of pulverized dry ice to the flask, wait until
all dry ice disappears, cover with the glass plate, wipe out the
moisture on the surface of the flask, and weigh.
3) With the flask left on the balance, monitor the weight every
minute up to 30 minutes.
4) Measure the temperature inside the flask.
5) Fill the flask with tap water and measure the volume of water.
Expt 2. Triple Point of Carbon Dioxide
1) Place clamp on both sides of the tygon tube and cover one side
with a nipple.
2) Using a funnel, fill the tube with pulverized dry ice so that about
5 cm length of the tube is filled.
3) Cover the other side with a nipple and tighten the clamp.
4) Wait until temperature rises and liquid carbon dioxide appears.
Rubbing the tube will speed up the temperature rise.
5) Once liquid carbon dioxide appears, release the pressure by
loosening clamp on one side and see what happens to the carbon
dioxide.
[Caution]
- Releasing the pressure after observing liquid CO2 could make a
big sound. Be prepared. The pressure should be released immediately
after reaching the triple point.
- Wear safety glass.
[Data Analysis]
1) To correct for the weight of air occupying the volume of the
flask, calculate the weight of air for the volume of the flask assuming
78% nitrogen, 21% oxygen, and 1% argon.
2) Calculate the weight of CO2 in the flask.
3) Calculate the molecular weight of CO2 by comparing the weight
of CO2 in the flask and the weight of air in the same flask.
4) Calculate the molecular weight of CO2 using PV = (W/M)RT.
Assume 1.0 atm pressure. (R = 0.082 L․atm/mol․K)
5) Discuss how the apparent molecular weight changes over time as
the flask is left open to the air.
[Additional Material]
Assignment for Internet Search
1. Gay-Lussac's Law of Combining Volume
Gay-Lussac (1778-1850)
2. Avogadro's Molecular Hypothesis