[washey]

Can someone give me specific details on how to work this formula? What number should I use for the MW, the one I came up with in my lab? What is the Volume-Percent they are referring to?

2. If you look up the composition of air, you will find that it is composed of:

N2, 78.084%
O2, 20.947%
Ar, 0.934%
CO2, 0.033%

There are some other gases present, but in very small quantities that do not influence its overall properties. The percentages shown above are by VOLUME.

Given the composition of air, it is possible to calculate a theoretical value for its apparent molecular weight as the weighted average of the individual components. The formula for this is:

MW(air) = (MW(1) * Volume-Percent(1) + MW(2) * Volume-Percent(2) + ...) / (100)

Note that the molecular weight of a component is for the entire molecule of the component, not the individual atom.

Calculate and record the theoretical molecular weight using the four components listed above.

[nj_bartel]

Each of those molecules occupies the same amount of space, so you'll be using the MW off the periodic table (remember they're molecules, not atoms) and the corresponding percentage of air.