TBR Gen Chem P4 #26

[dxu425]

The question presents an experiment where basically an aliquot of an unknown liquid hydrocarbon is placed inside a flask with a cap. The cap has a hole in it. The compound is heated to a boil, until vapor escapes through the hole in the cap. The liquid continues to boil until there's no liquid visible. It's then allowed to cool, whereupon the vapor inside the flask condenses into liquid. The liquid is massed (flask with liquid+cap minus empty flask+cap). It's assumed that when the heat source is removed, the flask is completely filled with vapor from the liquid and all of the air originally in the flask has been displaced.

SO the question (finally! haha) is: If the organic vapor had not fully displaced all of the air from the flask by the time the heat was removed, how would the results have been affected?

The answer is the mass of unknown liquid collected would be too small, so the calculated molecular mass would be too low. Explanation: if all the air hadn't been displaced by organic vapor, the actual amount of organic vapor would be less than the assumed value (100%), so the actual amount of liquid collected would be too low. If the measured mass of the liquid is too small, then the calculated molecular mass is too low as well.

What I don't understand is why the actual amount of organic vapor WOULD be less than 100%. It's not like you're collecting the vapor in a different flask after all the air has been displaced--in which case I'd understand why mistiming the initial collection of the vapor would be a problem, since you might be collecting some air as well. If the air in the flask isn't displaced, shouldn't you end up with the same amount of condensed liquid regardless? It's not that any of the vapor is disappearing, right?

I feel like I'm missing something really simple here. Thank you for your help!!

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[Erythropoietin]

Since you only have such a limited volume inside the flask, and if there's air already occupying some of that volume, then there will be less volume for the liquid vapor molecules to occupy. Since there's less of it, the calculated amount would also be less.

In the first case, though, all of the volume was occupied by the vapor. That's how I thought about it.

 

[bidiboom]

It's assumed that when the heat source is removed, the flask is completely filled with vapor from the liquid and all of the air originally in the flask has been displaced.

Though irrelevant from the question, but when the heat source is removed, air will be sucked in back with cooling, as long as the flask is not closed right before removing the heat source; thus the flask will never be filled with 100% hydrocarbon vapour.

 

[MT Headed]

What I don't understand is why the actual amount of organic vapor WOULD be less than 100%.

I feel like I'm missing something really simple here. Thank you for your help!!

If you accidentally put too little liquid in the flask in the first place, you would evaporate all your liquid but your hydrocarbon vapor would not occupy 100% of the flask volume.