Question

1.

A volatile organic liquid with known molecular weight is added to a vapor density flask which has a known volume. The flask is heated using a boiling water bath until all of the liquid has evaporated.
After heating for an additional 1 or 2 minutes, the flask is removed from the boiling water bath, corked, and cooled using a room temperature water bath. Predict the mass of condensed vapor.

Temperature of the hot water bath (ºC): 99.5

Volume of the flask (mL): 494.7

Molecular weight of the unknown (g/mol):   139.7

Barometer reading (mm Hg):   764.3

Calculate the mass of condensed vapor in g.

Answer 1

Q1. Barometer reading = 764.3 mmHg

atmospheric pressure = barometer reading = 764.3 mmHg

atmospheric pressure = 764.3 mmHg * (1 atm / 760 mmHg)

atmospheric pressure = 1.006 atm

pressure of unknown = atmospheric pressure

pressure of unknown = 1.006 atm

volume of unknown = volume of flask = 494.7 mL = 0.4947 L

Temperature of unknown = temperature of water = 99.5 ^oC = 372.5 K

According to ideal gas law,

moles of unknown = [(pressure of unknown) * (volume of unknown)] / [(R) * (temperature of unknown)]

where R = gas constant = 0.0821 atm-L/mol-K

moles of unknown = [(1.006 atm) * (0.4947 L)] / [(0.0821 atm-L/mol-K) * (372.5 K)]

moles of unknown = 0.01627 mol

mass of unknown = (moles of unknown) * (molar mass of unknown)

mass of unknown = (0.01627 mol) * (139.7 g/mol)

mass of unknown = 2.27 g