I did an experiment with a volatile liquid to determine its molecular mass from its vapor density. To better weigh it with less of a vapor pressure error, would it be better to chill flask and condensate in a fridge before weighing it?

The flask was in a hot water bath where we measured the temperature of the boiling water which is said to be the temperature of the gas. Wouldn't it instead give an inaccurate reading of the gas? because it wouldn't be aligned with the reading of the water temperature? am I getting any where with understanding this concept because I feel like it would be incorrect to chill the flask but I can't understand exactly why.

1 Answer
Oct 28, 2017

If you were to pre-chill the volatile liquid, your initial pressure reading may be inaccurate depending on how long it takes for the liquid to reach the water temperature.
Do not pre-chill.


To perform this experiment properly, you will need to know the
exact temperature of the gas at the time that the pressure reading(s) are being observed. That means the entire volume of the liquid must reside at the water temperature for the length of time the measurements are being made.

Pre-chilling the liquid may add to the stabilization time required for this experiment. If you are submerging chilled liquid containers into boiling water, they would have to be metal (stainless steel) and they would equalize the temperature of the liquid to the water very quickly, regardless. It would be dangerous to immerse chilled glass containers into boiling water.

This is an example of the experiment: