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Topic: Need help on a ACS Local Exam question (determing molar mass)  (Read 4924 times)

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Offline xangelofxdeathx

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this is number 5 from ACS 2001 Local exam.

The molar mass of a volatile liquid (boiling point < 90degree C) is to be determined by measuring the density of its vapor in an Erlenmeyer flask capped with Al foil with a pinhole to allow the vapor to escape . A sample of the liquid is added to the pre-weighed flask that is heated in a water bath until the liquid has evaporated, after which the flask is dried and reweighed. Which piece of information is not required to determine the molar mass of the liquid?

A) barometric pressure
B) mass of liquid sample
C) temperature of water
D) volume of the flask

My question: (The answer is B) Since the molar mass is equal to the density times the gas constant times the temperature all over pressure. Wouldn't you need all of these components? (A) gives you the pressure, (B) gives you the mass needed to calculate the density, (C) gives you the temperature, and (D) gives you the volume needed to calculate the density. I'm confused how you would be able to find the density of the liquid if you don't know the mass of the liquid sample.  ??? Could someone explain to me why the answer is B?

Offline DevaDevil

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Re: Need help on a ACS Local Exam question (determing molar mass)
« Reply #1 on: March 28, 2007, 02:10:54 AM »
gaslaw:
p V = n R T

so you need pressure, volume and temperature. this will give you the number of moles.
since you also have the weight (difference between before and after evaporation minus the weight of the air that was in there during preweighing - assuming the flask is filled with the vapor after evaporation) you then have weight / moles = molar mass :)

Offline xangelofxdeathx

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Re: Need help on a ACS Local Exam question (determing molar mass)
« Reply #2 on: March 28, 2007, 02:15:15 AM »
Oh I see, cool thank you  ;D

Offline enahs

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Re: Need help on a ACS Local Exam question (determing molar mass)
« Reply #3 on: March 28, 2007, 09:59:13 AM »
I made a joke that was wrong, I suck.

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