The passage i'm referring to is Passage XI (questions 71-76) from Section VII (Phases & Phase Changes), of TBR General Chemistry Part II.
For those of you without the book:
The Dumas experiment involves filling a flask of known mass and volume with an organic liquid. The organic liquid is heated, so that it begins to vaporize. The air is displaced out of the flask by the organic vapors through a small hole at the top of the flask. The solution is heated by flame until the last trace of liquid has evaporated.
Once the liquid has completely evaporated, it is assumed the flask is filled entirely with organic vapors. As the flask cools, the vapors condense back into a liquid, and air flows back into the flask through the small hole. Once the flask is back to its initial temperature, the combined mass of the flask and condensed organic liquid is measured. The increase in mass over the initial weighing of the empty flask can be attributed to the organic liquid in the flask. The small amount of air displaced by the organic liquid is assumed to be negligible.
The mass of the organic liquid can be used to determine the molecular mass of the unknown organic liquid. Because the pressure, temperature, and volume of the gas in the flask are known at the boiling point of the liquid, the moles of the liquid in the flask can be determined using PV = nRT.
I am trying to understand this experiment, and the purpose of each step. Will V (in the equation PV = nRT) be the volume of the flask, which I assume they know beforehand? And how do they know the pressure of the vapor, or the temperature; the passage doesn't mention any instruments like a barometer or a thermometer.
Finally, to find the mass of the organic liquid, why don't they just place a beaker on a scale, zero the scale, and add the mass to the beaker? Or why didn't they weigh the flask before burning the organic liquid, and compare that weight to the empty flask, rather than weighing the liquid after vaporizing and condensing it?
For those of you without the book:
The Dumas experiment involves filling a flask of known mass and volume with an organic liquid. The organic liquid is heated, so that it begins to vaporize. The air is displaced out of the flask by the organic vapors through a small hole at the top of the flask. The solution is heated by flame until the last trace of liquid has evaporated.
Once the liquid has completely evaporated, it is assumed the flask is filled entirely with organic vapors. As the flask cools, the vapors condense back into a liquid, and air flows back into the flask through the small hole. Once the flask is back to its initial temperature, the combined mass of the flask and condensed organic liquid is measured. The increase in mass over the initial weighing of the empty flask can be attributed to the organic liquid in the flask. The small amount of air displaced by the organic liquid is assumed to be negligible.
The mass of the organic liquid can be used to determine the molecular mass of the unknown organic liquid. Because the pressure, temperature, and volume of the gas in the flask are known at the boiling point of the liquid, the moles of the liquid in the flask can be determined using PV = nRT.
I am trying to understand this experiment, and the purpose of each step. Will V (in the equation PV = nRT) be the volume of the flask, which I assume they know beforehand? And how do they know the pressure of the vapor, or the temperature; the passage doesn't mention any instruments like a barometer or a thermometer.
Finally, to find the mass of the organic liquid, why don't they just place a beaker on a scale, zero the scale, and add the mass to the beaker? Or why didn't they weigh the flask before burning the organic liquid, and compare that weight to the empty flask, rather than weighing the liquid after vaporizing and condensing it?
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