Question

Some liquids have enough attractions between molecules to form dimers. (Dimers are molecules formed from the combination of identical molecules, A + A --> A2.) What effect would this have on the experimental molar mass?
This was a molar mass lab. Alcohol evaporated in an e-flask and the vapor then condensed and massed.

Answer 1

This would definitely have an effect upon the molar mass determined. I have done this with acetic acid (molar mass = about 60) and found the molar mass to be 90 which shows about half of the molecules had dimerized.

Answer 2

what type of bond holds the dimer together? If it is a hydrogen bond, then wouldn't it separate back to A +A in the vapor phase and then have the same molar mass. This is what I am struggling with.

Answer 3

This is because this is the effect on the EXPERIMENTAL molar mass, which depends only on P, V, and mass. The actual molar mass stays the same; just the molar mass that you calculate changes once the molecules start forming dimers.