AP Chemistry
Experiment – Molar Mass of a Volatile Liquid
Objectives
To observe the gaseous state of matter and determine the molar mass of
a volatile liquid.
Introduction
To identify a new compound, a chemist must first identify its
physical properties. These properties include: melting point, color,
density, and elemental composition. In this experiment we will determine
the molar mass of an organic compound.
A number of methods can be used to determine the molar mass of
a compound. In the case of compounds with large molecules such as
proteins, drugs, and enzymes, an osmometer is used. In the case of
smaller a mass spectrometer is used. For the case of volatile liquids as in
the case of our lab the method developed by John Dumas (1800 – 1884)
will be used. The unknown liquid in question is placed in a small flask of
known volume. The flask is then placed into a beaker of boiling water.
The liquid vaporizes and the number of moles is then calculated. The
basic assumption of a ideal gas will be the basis of calculation. The
pressure will be set to atmospheric.
PV  nRT
Accuracy is very important to obtain good results in this experiment.
The mass of the vapor can be determined by the following relation:
mvapor  m flask  vapor _ m flask
The molar mass of the compound can be determined by the following
relation:
m
M compound  vapor
nvapor
Gases and liquids with relatively large intermolecular forces and volumes
do not follow the ideal gas laws or Kinetic Molecular Theory. Some
compounds such as water deviate significantly from the ideal gas law.
Under these conditions Van der Waal’s equation can be used in place of
the ideal gas law. This equation accounts for intermolecular forces.
n2 a
( P  2 )(V  nb)  nRT
V
The values for P, V, n, R, and T are the same as in the ideal gas law. The
values for “a” and “b” represent constants accounting for intermolecular
forces. These values are expressed in the following table for some
liquids.
1
Van der Waal Constants for some Low Boiling Point Liquids
Name
Methanol
Ethanol
Acetone
Propanol
Hexane
Cyclohexane
Pentane
Water
a
L2  atm
mole 2
9.523
12.02
13.91
14.92
24.39
22.81
19.01
5.46
b
L
mole
0.06702
0.08407
0.0994
0.1019
0.1735
0.1424
0.1460
0.03051
Boiling Point
Degrees C
65.0
78.5
56.5
82.4
69.0
80.7
36.0
100.0
Pre Lab Questions
Answer the following questions on another sheet of paper.
1.
Explain how the mass and vaporized liquid is determined by this
experiment?
2.
Define molar mass.
3.
A .149 gram sample of un known vapor occupies a flask at 89
degrees C, 771 torrs, and a volume of 148 ml. Assume Ideal Gas
Behavior. How many moles of the vapor are present? What is the
molar mass of the vapor?
4.
Explain the meaning of the values for “a” and “b”.
Experimental Procedure
1.
Determine the total volume inside the flask by filling it with water to
the brim and then measuring the volume of water. Prepare a 125
ml flask by thoroughly cleaning it and drying it. Cover the end of
the flask with a piece of Aluminum Foil. Hold the foil in place with a
rubber band.
2.
Measure the mass of the flask with rubber band and foil.
3.
Uncover the flask and place 5 ml of unknown liquid inside the flask
and place the foil over the end of the flask, secure the foil with the
rubber band.
2
4.
Using a 600 ml beaker add enough water to act as a heating bath
so that when the flask is inserted, enough space remains to allow
proper boiling of water. See instructor.
5.
Puncture several pin size holes through the foil to allow the gas
inside to always be at atmospheric pressure.
6.
Gently heat the beaker to boiling. Be sure you have only a gentle
boiling of the water.
7.
Constantly monitor the temperature of the water.
8.
When all of the unknown liquid appears to disappear from the flask,
remove it from the beaker. Allow the flask to cool be placing it in
ice.
9.
Dry the outside of the flask carefully and measure the mass of the
flask, foil, and unknown gas.
10.
Perform this experiment 3 times to ensure accurate data.
Data Sheet
Mass of flask, rubber band, and foil
________ ________ ________
Temperature of boiling water
________ ________ ________
Mass of dry flask +
foil assembly
________ ________ ________
Volume of flask
________ ________ ________
Atmospheric Pressure
________ ________ ________
Calculations
Amount of vapor nvapor
________ ________ ________
Mass of vapor mvapor
________ ________ ________
Average molar mass
________________
Unknown liquid
_________________________
3
Standard deviation
Show calculations for one trial + error analysis
Show all calculations for all trials on another sheet of paper.
Post Lab Questions
Answer the following questions on another sheet of paper.
1.
What would be a suitable hypothesis for this experiment?
2.
What would be a suitable conclusion for this experiment?
3.
Explain all your errors.
4.
Can the assumption that the unknown vapor in the flask is an Ideal
Gas be used? Why/Why not?
5.
Can this technique be used to find the molar mass of all alcohols?
Why/Why not?
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