When determining the molar mass of a volatile liquid by heating the liquid and then using the condensed, volatile vapor's mass, why do volatile liquids with lower boiling points give better results?
Also, what effect would the formation of dimers have on the experimental molar mass?
Thank you~
When determining the molar mass of a volatile liquid by heating it and measuring the mass of the condensed vapor, volatile liquids with lower boiling points generally give better results for a couple of reasons.
Firstly, volatile liquids with lower boiling points tend to have weaker intermolecular forces compared to liquids with higher boiling points. This means that the molecules of the lower boiling point liquid are more likely to vaporize and condense without undergoing significant chemical reactions or forming aggregates. As a result, the measured mass of the condensed vapor will be closer to the actual molar mass of the individual molecules.
Secondly, volatile liquids with lower boiling points are less likely to form dimers or other aggregates in the vapor phase. Dimers are formed when two molecules of the same substance bind together, typically through intermolecular forces such as hydrogen bonding or van der Waals interactions. The formation of dimers affects the measured molar mass because the condensed vapor will contain both individual molecules and dimers. This mixture will give a higher average mass compared to a vapor composed solely of individual molecules. Consequently, the presence of dimers introduces an additional complication in determining the accurate molar mass of the volatile liquid.
To mitigate the effects of dimer formation and improve the accuracy of the experimental molar mass determination, it is generally preferable to work with volatile liquids that have lower boiling points. This increases the likelihood of measuring the molar mass of individual molecules rather than an average mass influenced by dimer formation. However, it's worth noting that the presence of dimers can still occur in volatile liquids with lower boiling points, especially if the molecular structure or intermolecular forces dictate dimer formation. In such cases, additional techniques or modifications to the experiment may be necessary to obtain accurate molar mass measurements.