Molecular Weight of a Volatile Liquid

In this experiment, you will be determining the molecular weight of a volatile liquid. A sample of gas will be introduced into a container of known volume. The temperature and pressure of the system will also be determined. From this data, using the ideal gas law, you can determine the number of moles of the substance and knowing the mass of sample introduced, the molecular weight can be determined.

Procedure (sortof):
The substance is an ether, which is a class of organic molecules that are very volatile. You will connect a pressure sensor to a sealed container (of which you measure the volume of after you are all done) Using a syringe, you will remove some of the air in the system to drop the internal pressure of the system. Then fill a smaller syringe with a an amount of ether and measure the mass of the syringe (for weighing purposes, stick the syringe into a rubber stopper.) Using that syringe, inject an amount of ether into the flask and re-weigh the syringe) The mass of ether can easily be ascertained. The pressure sensor will measure the increase in pressure as the liquid turns to a gas. A pressure reading is taken when the measured pressure levels off and the liquid is all evaporated (make sure)  There was an initial pressure of 'air' in the flask that is known (Pair) which is simply the pressure reading before the start of the experiment) The final (stabilized) pressure represent the total pressure. The pressure coming from the unknown can easily be determined from the relationship Ptotal = Pair + Punk. With Punk in hand as well as T (room T), volume (what you measure after all runs are done) you can calculate nunk. The molecular weight can then be calculated.

Short form memo lab report  Last one of the term, promise. Make sure to include average molecular weight, standard deviation and %error.

Prelab questions:
(to be completed on a separate piece of recycled paper, handed to the instructor as you enter lab)

1. An experiment was done and found that the mass of inserted vapor was 0.2193 g. The volume of the flask was determined to be 248.34 mL. The pressure before the liquid (which evaporates) gas was inserted was 340.4 torr. After the liquid was introduced, the pressure (because it evaporated) went up to 803.4 torr. The temperature of the day was 23.3 C.

(a) Calculate the number of moles of vapor inside the flask.
(b) Calculate the molecular weight of the unknown substance.

2. Make a chart to record your data for the procedure.

3. Research the MSDS for dimethyl ether and diethyl ether. Give a brief overview of their hazards.

~MEO 6 Dec 2010