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.
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.
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.
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.
In a molar mass lab where alcohol is being evaporated and condensed, the presence of dimers can affect the experimental molar mass.
When some liquids form dimers, the vapor that is being condensed will consist of both monomers (single individual molecules) and dimers. Since dimers are made up of two identical molecules, their molar mass is twice that of a single monomer.
As a result, if there are dimers present in the vapor that condenses, the molar mass calculated experimentally would be higher than the actual molar mass of the alcohol. This is because the weight measurement includes both monomers and dimers, leading to an overestimate of the molar mass.
To account for this, it's essential to consider the equilibrium between monomers and dimers in the vapor phase. If the equilibrium is known, it is possible to calculate the fraction of dimers present and correct the experimental molar mass accordingly.
One way to determine the equilibrium constant of dimerization is by studying the temperature dependence of vapor pressure using the Clausius-Clapeyron equation. By measuring the vapor pressure at different temperatures, one can determine the equilibrium constant and then calculate the fraction of dimers present.
Overall, the presence of dimers in the vapor phase can impact the experimental molar mass and needs to be taken into account when analyzing the results of the molar mass lab.