Purpose
In this experiment we will determine the molecular weight of the gas used in our lab Bunsen burners.  We will do this by comparing the mass of a known volume of burner gas to the mass of an identical volume of air under the same conditions.

Pre-Lab
The molecular weight of air is the weighted average of the molecular weights of the gases that make up air.  Dry air is made up of nitrogen (78.084%), oxygen (20.948%), argon (0.934%), carbon dioxide (0.0315%), neon (0.00182%), and helium (0.00052%).  There are trace amounts of other gases that we can ignore, and a varying but substantial amount of water which we will ignore for simplicity.  Determine the molecular weight of dry air.

According to Avogadro’s Hypothesis, equal volumes of different gases under identical conditions contain equal numbers of molecules.  If a volume of gas has twice the mass of the same volume of air, then each molecule must have twice the mass of each “molecule of air” and therefore twice the molecular weight.

Procedure: Mass of A Gas

1. Assemble a bag, stopper, rubber band, and medicine dropper as demonstrated.
2. Roll the bag tightly shut, and determine the mass of the assembly with no air inside.
3. Roll the bag tightly to squeeze out all of the air.  Fill the bag with gas from a nozzle at one of the lab stations.  The bag should be filled completely, but not tightly.  Cap the bag.  Determine and record the mass of the bag filled with burner gas.
4. Use an air pump to inflate the bag with air and recap it.  Determine and record the mass of the bag filled with air.  (Take note of how this mass compares with that of the empty bag.)
5. Fill a large graduated cylinder with water and invert it into a pan filled with water.  Determine the volume of the filled bag by connecting a hose from the bag up into the graduated cylinder, and squeezing all the air in to displace the water.  Determine and record the volume of the bag.
6. Record the pressure and temperature in the room.
7. Use the recorded pressure, temperature and volume of air to determine the moles of the air in the bag.  Then calculate the mass of air in the bag under room conditions.
8. Just like in water, an object submerged in air is buoyed up by a force equal to the weight of air displaced.  This effect is small enough to be ignored in most circumstances, but it is important when determining the measured mass of a gas.  The mass of burner gas obtained in step three is actually too low by the mass of air obtained in step seven.  Calculate and record the actual mass of burner gas in the bag.
9. From the adjusted mass of burner gas, the calculated mass of air, and the molecular weight of air, determine the molecular weight of burner gas.
10. Decide whether the burner gas is most likely CH₄, C₂H₆, or C₃H₈.  Explain the basis for your decision.

The gas in the burner is actually a mixture of two of the gases listed above.  There are three possible pair combinations.  Given the calculated molecular weight, what is the percentage of each gas in each possible mixture?

Pass In
For this lab you will pass in a LEGIBLE sheet of paper with the following:

• A heading that includes your name, your partner’s name, the date, and the title of the lab.
• A data table that includes every measured quantity.  Remember that every value should be described in English AND labeled with the proper units (for example,  Mass of Cheeseburger  118.2 g )
• A calculations/results section that describes and clearly shows every calculation.  Make sure I know what you are thinking.
• A decision about which formula most likely applies to the burner gas.
• A discussion that includes any opinion about the lab or results, any possible errors, and any suggestions for getting better results.