The Relative Masses of Gases

CAUTION!

This demonstration uses equipment under vacuum. Because of the danger of implosion it must be carried out only by those properly trained in handling evacuated glass containers.


A safety shield must be in place at all times, and hand and eye protection must be worn.

It must be carried out as a demonstration, not as an experiment conducted by students.

Materials:

  1. 500 mL round bottom flask
  2. one hole rubber stopper
  3. vacuum stopcock
  4. vacuum pump
  5. source of gases such as oxygen, carbon dioxide and methane (you can use natural gas as a source of relatively pure methane gas)
  6. balance or scale capable of measuring the mass of the assembled flask and stopper to the nearest 0.01 g

Procedures:Flask

  1. Carefully insert the stopcock into the rubber stopper, and place the stopper into the flask as shown in the diagram.
  2. Attach to a vacuum pump with a length of rubber tubing, and evacuate the flask. Close the stopcock and disconnect the rubber tubing. Find the mass of the empty flask. Use extreme caution not to bump the flask against any hard object while it is evacuated.
  3. Connect the closed flask to a source of dry compressed gas. Using the regulator on the compressed gas, fill the flask to just slightly above atmospheric pressure, and then disconnect the flask. Leave the stopcock on the flask open, so that the pressure inside the flask drops to atmospheric, and find the mass. Use caution not to overfill the flask, or the pressure will blow out the stopper.
  4. Repeat steps 2 and 3 for each available compressed gas.
  5. Complete the calculations necessary to fill out the following data table. Since the flask is always at the same temperature and pressure, it should always have the same number of molecules of each gas in it. This allows us to calculate the relative mass of each gas compared to a standard. In this case the standard selected is Oxygen at a molar mass of 32. If we multiply this value by the relative mass, we will have an experimental molar mass for each gas.

 

Gas Used Mass of Flask
Assembly
and Gas (g)		 Mass of Empty
Flask Assembly
(g)		 Mass of Gas
in Flask
(g)		 Relative Mass
Compared
to O2		 Experimental
Molar Mass
Oxygen ? ? ? 1.0 32
Carbon Dioxide ? ? ? ? ?
Methane ? ? ? ? ?

Discussion:

  1. Are the conditions for Avogadro's hypothesis being followed closely enough in this experiment that we can assume that there are the same number of molecules in the container each time?
  2. Why does assuming there are the same number of molecules in the container each time allow you to calculate the molar mass?
  3. What are some of the main sources of systematic error in this experiment?

Alternative Procedure:

If you do not have a vacuum stopcock, then you can improvise one using a length of glass tubing, and a short (10 cm) piece of light rubber tubing (do not use thick wall vacuum tubing). Put the glass tubing through the stopper, and attach the rubber tubing to the other end. Attach the vacuum pump to this rubber tubing (you will need a second length of glass tubing to join the hose from the vacuum pump to this flexible tubing). When the flask has been evacuated, pinch the flexible tubing into a tight bend, and clamp it off with a pinch clamp.

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Copyright © 1998 - 2008 David Dice