The molar mass of a volatile liquid(one which vaporizes easily) can be determined by a the 'Dumas Method.' In this process a small amount of the liquid (a few drops) is placed in a clean flask. The mouth of the flask is covered with a piece of foil that has a small hole (pin hole) in it. The flask is heated in a boiling water bath until the liquid vaporizes completely. The air in the flask (and the excess vapor) escapes through the pinhole leaving the flask filed with the vapor. When the flask cools the vapor condenses and can be massed along with the flask. In this way the volume of the vapor(volume of the flask),temperature of the vapor(temperature of the water bath) pressure of the vapor (atmospheric pressure) and mass of vapor (mass of flask and condensed vapor minus the mass of the empty flask) can be determined. The molar mass of the gas can then be determined using the formula from this lab. Suppose that the original few drops of liquid added to the flask contained a nonvolatile (will not vaporized) contaminate. What effect (if any) would this have on the experimentally determined molar mass of the liquid?
flask + liquid = ??
flask + vapor = ??
empty flask = ??
flask + liquid + contam = ??
flask + vapor + contam = ??
empty flask ??
This may help you find your answer.
If the original few drops of liquid added to the flask contained a nonvolatile contaminant that will not vaporize, it will have an effect on the experimentally determined molar mass of the liquid. The presence of a nonvolatile contaminant means that the molar mass of the contaminant will also be included in the final mass of the vapor obtained from the Dumas Method.
When the flask is heated in the boiling water bath, only the volatile liquid will vaporize and escape through the pinhole, leaving behind the nonvolatile contaminant in the flask. During the cooling process, the vapor condenses along with the nonvolatile contaminant, and both will be massed together with the flask.
Since the nonvolatile contaminant does not vaporize, its mass will be included in the final mass of the vapor. This means that the experimentally determined mass of the vapor will be higher than it would be if the liquid did not contain any nonvolatile contaminants.
As the molar mass is calculated using the mass of the vapor, the presence of the nonvolatile contaminant will lead to an overestimation of the molar mass of the volatile liquid. The contaminant's additional mass will contribute to the overall mass of the vapor, resulting in a higher calculated molar mass.
To obtain an accurate molar mass of the volatile liquid, it is important to ensure that the liquid is free from nonvolatile contaminants before beginning the Dumas Method.