Measuring the Molar Mass of an "unknown" gas

Teacher notes:

The purpose of doing the procedure in this way (using the syringe) is to avoid getting the lighter wet. The traditional way of doing this experiment is to place the lighter in the water, and let the gas rise into the eudiometer. However, this means you need to somehow dry the lighter – a difficult task in a one period class.
If you don't have a barometer you can get the air pressure from a weather station – BUT YOU HAVE TO ASK FOR THE ACTUAL STATION PRESSURE. Unless you happen to live at sea-level, what the weather office will give you is an altitude corrected value. The formula to do this is complex. However, they will know the real pressure because its needed for setting airplane altimeters. You just have to ask the right question of the the weather people and you can usually get the right value.
The mass of gas is quite small. You really need a balance capable of measuring mg or else you will have to use a much larger eudiometer. With a 50 mL eudiometer and a mg balance, you will get 2 significant digits. If you only have a cg balance, you will have to use a 250 mL graduated cylinder, to get the same level of accuracy.
Modify the lighter by gluing a short piece of vinyl tubing onto the valve. Use a hot glue gun, put a little glue on the tubing, and hold it in place until cool. This extension is necessary to allow the plunger of the syringe to press on the valve. Make sure that you get no glue on the other parts, or the valve will be stuck (press the tubing and valve up and down while the glue is cooling to make sure this doesn’t happen).
A mini lighter will fit inside a 35 mL syringe. If you use a standard size lighter, you will have to use a larger syringe.
Step 7 in the procedure is important (measuring the original position of the plunger before gas is released). The students need to know this position so that they can return the plunger to this position after the gas is released, but before they pull the tubing out of the eudiometer. This accounts for the volume change because of the change in position of the plunger. Make sure that they pull the plunger back to its original position (step 8) with the tubing high enough in the eudiometer that they will not draw water back into the syringe.
Students love to light the gas on fire. It’s not a good idea. There should be virtually no oxygen in the eudiometer, so the flame burns at the top. However, it can get to be a pretty big flame, and there is potential for a burn, or worse, air getting mixed in the eudiometer and causing an explosion.
Major sources of systematic error in this experiment are reproducing the original plunger level, and trying to read the level of the gas in the eudiometer while the water levels are equal. If you don’t mind the mess, one solution is to completely fill the pail with water at the end of the experiment. It is then easier to adjust the levels, and to read the eudiometer level. An alternative is to use a modified acid-base buret as a water-filled manometer, as shown in this diagram. A large bore needle is placed through a rubber stopper and the water levels adjusted to the 0.0 point on the buret. When the levels are adjusted, attach the syringe to the Luer lock on the needle. You can test for a gas tight setup by lowering the funnel (the levels will change, but become stable). The same precaution about reading the original plunger level and then restoring it to this level at the conclusion must be followed.

It is fairly easy to lower the funnel to get the water level the same in the eudiometer and funnel side before reading the volume.

Butane dissolves in rubber latex tubing. It is less of a problem if you use vinyl tubing. It also probably dissolves slightly in the soft rubber of the plunger. At the end of the experiment, disassemble the syringes to allow the butane to evaporate.