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?

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.

DrBob222 · Accepted Answer

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.

Anonymous · Answer

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.

Ashwin · Answer

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.