Molecular mass of a volatile liquid sources of error?

Answer 1

This method depends on a lot of things going right. One assumption is that while the liquid is volatile enough to vaporize at the elevated temperature, it is not so volatile that a significant amount will be lost to evaporation through the orifice as the container cools. The vapor is also assumed to behave ideally at the temperature and pressure at which it occupies the container. The amount of error implicit in this approximation varies from compound to compound and is tied to the variables which create deviations from ideal behavior: molecular volume and intermolecular forces. Generally speaking, the larger these are, the greater the error in the determination. The situation is further complicated by the interaction of these two factors. For example, a small molecule may have significant intermolecular forces (hydrogen bonding perhaps) but a large molecule may have comparatively weak forces (like dispersion forces). Combinations run between these extreme examples making predictions of error from this source difficult.

There is another problem with the basic method which is more easily addressed. When the container is "empty", i.e., before the liquid is added or the vapor forms, there is a fixed amount of air determined by the volume, temperature and pressure in the container. This air has mass and contributes to the overall mass of the container assembly. But when the liquid is present it vaporizes to some extent determined by its vapor pressure at room temperature. The presence of this vapor forces out some of the air through the orifice since the internal pressure is equalized with the room pressure and the number of gas particles remains constant if the temperature and pressure remain constant. Thus when the container is massed again there is air missing that should be counted in the final mass if the mass of the condensed liquid is to be determined by difference. This results in an apparent mass of liquid that is too small which in turn makes the molar mass calculation too small.

Answer 2

Sources of error in determining the molecular mass of a volatile liquid could include incomplete vaporization of the liquid, loss of sample during transfer, improper calibration of equipment, and variations in atmospheric pressure affecting the boiling point determination. These errors can lead to inaccuracies in the calculated molecular mass.