TBR Gen Chem chapter One passgae4 question 26

[jamilabella20]

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Hi
i didnt get the answer to the TBR question 26 of passage4 chapter 1 in Gen chem,
if we didnt displace all of the air in the flask, i do understand that the purity of the solution will decrease but wont the volume measured be increased ?
and i dont get how is it related to molecular Mass?
bc moles= Volume/Volume(STP), so if Volume increases then Moles will increase as well. then we have moles= mass/molecular weight bc they are inversely prop then when moles increase the molecular weight decrease
please help me
thanks

 

[BerkReviewTeach]

The glass flask has a finite volume that does not change with temperature.

The point of this experiment is that you can't get the molecular mass of an unknown liquid easily, but if you convert it to a gas, then you can at least get the moles using PV=nRT. In this experiment, the liquid is heated until it completely evaporates, which should result in a flask of known volume with a pure vapor at its boiling point with a pressure equal to atmospheric (it's an open system). Using n = PV/RT, you can determine the moles of gas in the flask under those conditions.

When you condence the vapor back into a liquid at the original temperature, air fills the flask once again (along with the condensed vapor at the bottom). By taking the mass of the system and subtrating the first mass (cap and flask with air inside), you get the mass of the liquid, which happens to equal the mass of the vapor.

When you divide the mass of the vapor by the moles, you get the molecular mass of the unknown compound. So the point of the question is that in the event there is some air left in the flask, then the mass you determine in the end is too small. However, the volume of the rigid flask is the same, the temperature at boiling is still the same, and the atmospheric pressure is still the same. So you do the same calculation for moles, and thereby have the same denominator.

This leads to an error, which is the crux of this question.

Hope that clears it up.