a super PLAYCHEM experiment
Calculating the Molecular Weight of an UNKNOWN GAS
using Avogadro's Principle
You've heard of his number, now apply his principle!

In 1807, Amedeo Avogadro postulated the following:
Equal volumes of gas, at the same temperature and pressure, contain equal number of  molecules.

In this experiment, you will use this concept to calculate the molecular weight of a computer assigned gas.

What you will do is weigh a flask and stopper. At this point the flask is filled with air, a mixture mainly of N2, O2, H2O and Ar, with an  average MOLECULAR WEIGHT of about 29.

avog1.jpg (7353 bytes)


You will next fill the flask with the assigned gas and stopper tightly.  The exact technique will depend on whether the gas is more dense or less dense than air.

If the filling technique shown on the left is correct, is the MW of the unknown gas greater or less than 29?

The stoppered flask filled with "unknown gas" is weighed. This is the second weighing.

In this experiment you will make weighings using the Electronic Analytical Balance.  This will rapidly weigh to 0.1 mg or .0001 g.

It is best to confirm the weight of the stoppered flask filled with gas by refilling it and reweighing.   These duplicate measurements should agree within about 1-3 mg.

You will make a third weighing.  The mass of the flask "filled" with distilled water and the stopper.
You should also note the atmospheric pressure and temperature.

avog2.jpg (10833 bytes)
From these DATA, you should be able to:
  • Calculate the volume of the gas contained in the flask. You may assume the density of water is 0.996 g/cc
  • Using the ideal gas law, you will be able to calculate the mass of air contained by the flask.
  • Finally you can calculate the Molecular Weight of the unknown gas.

RW Kluiber 1/12/2000